Lemma 1
Let \(\xi :\triangle :=[\eta _{1},\eta _{2}]\times [ \vartheta _{1}, \vartheta _{2} ] \subset \mathbb{R} ^{2}\rightarrow \mathbb{R} \) be a partial differentiable mapping on \(( \eta _{1},\eta _{2} ) ]\times ( \vartheta _{1}, \vartheta _{2} ) \). If \(\frac {\partial ^{2}\xi (t,s)}{\partial t\partial s}\in L_{1}( \Delta )\), then the following identity holds:
$$\begin{aligned}& \xi \biggl( \frac{\eta _{1}+\eta _{2}}{2}, \frac{\vartheta _{1}+\vartheta _{2}}{2} \biggr) + \frac{2^{\gamma _{1}\alpha -1}2^{\gamma _{2}\beta -1}\Gamma (\alpha +1)\Gamma (\beta +1) \gamma _{1}^{\alpha }\gamma _{2}^{\beta }}{ ( \eta _{2}-\eta _{1} ) ^{\gamma _{1}\alpha } ( \vartheta _{2}-\vartheta _{1} ) ^{\gamma _{2}\beta }} \\& \qquad {}\times \biggl[ ^{\gamma _{1}\gamma _{2}}I_{\eta _{1}^{+},\vartheta _{1}^{+}}^{ \alpha ,\beta }\xi \biggl( \frac{\eta _{1}+\eta _{2}}{2},\frac{\vartheta _{1}+\vartheta _{2}}{2} \biggr) \\& \qquad {}+ ^{\gamma _{1}\gamma _{2}}I_{\eta _{1}^{+},\vartheta _{2}^{-}}^{ \alpha ,\beta }\xi \biggl( \frac{\eta _{1}+\eta _{2}}{2}, \frac{\vartheta _{1}+\vartheta _{2}}{2} \biggr) +{}^{\gamma _{1} \gamma _{2}}I_{\eta _{2}^{-},\vartheta _{1}^{+}}^{\alpha ,\beta } \xi \biggl( \frac{\eta _{1}+\eta _{2}}{2},\frac{\vartheta _{1}+\vartheta _{2}}{2} \biggr) \\& \qquad {}+{}^{\gamma _{1}\gamma _{2}}I_{\eta _{2}^{-},\vartheta _{2}^{-}}^{ \alpha ,\beta }\xi \biggl( \frac{\eta _{1}+\eta _{2}}{2}, \frac{\vartheta _{1}+\vartheta _{2}}{2} \biggr) \biggr] -A \\& \quad = \frac{\gamma _{1}^{\alpha }\gamma _{2}^{\beta } ( \eta _{2}-\eta _{1} ) ( \vartheta _{2}-\vartheta _{1} ) }{16} \\& \qquad {}\times \biggl\{ \int _{0}^{1} \int _{0}^{1} \biggl[ \frac{1}{\gamma _{1}^{\alpha }}- \biggl( \frac{1-(1-t)^{\gamma _{1}}}{\gamma _{1}} \biggr) ^{\alpha } \biggr] \biggl[ \frac{1}{\gamma _{2}^{\beta }}- \biggl( \frac{1-(1-s)^{\gamma _{2}}}{\gamma _{2}} \biggr) ^{\beta } \biggr] \\& \qquad {}\times \frac {\partial ^{2}\xi }{\partial t\partial s} \biggl( \frac{1+t}{2}\eta _{1}+\frac{1-t}{2}\eta _{2},\frac{1+s}{2}\vartheta _{1}+ \frac{1-s}{2}\vartheta _{2} \biggr)\,ds\,dt \\& \qquad {} - \int _{0}^{1} \int _{0}^{1} \biggl[ \frac{1}{\gamma _{1}^{\alpha }}- \biggl( \frac{1-(1-t)^{\gamma _{1}}}{\gamma _{1}} \biggr) ^{\alpha } \biggr] \biggl[ \frac{1}{\gamma _{2}^{\beta }}- \biggl( \frac{1-(1-s)^{\gamma _{2}}}{\gamma _{2}} \biggr) ^{\beta } \biggr] \\& \qquad {}\times \frac {\partial ^{2}\xi }{\partial t\partial s} \biggl( \frac{1+t}{2}\eta _{1}+\frac{1-t}{2}\eta _{2},\frac{1-s}{2}\vartheta _{1}+ \frac{1+s}{2}\vartheta _{2} \biggr)\,ds\,dt \\& \qquad {} - \int _{0}^{1} \int _{0}^{1} \biggl[ \frac{1}{\gamma _{1}^{\alpha }}- \biggl( \frac{1-(1-t)^{\gamma _{1}}}{\gamma _{1}} \biggr) ^{\alpha } \biggr] \biggl[ \frac{1}{\gamma _{2}^{\beta }}- \biggl( \frac{1-(1-s)^{\gamma _{2}}}{\gamma _{2}} \biggr) ^{\beta } \biggr] \\& \qquad {}\times \frac {\partial ^{2}\xi }{\partial t\partial s} \biggl( \frac{1-t}{2}\eta _{1}+\frac{1+t}{2}\eta _{2},\frac{1+s}{2}\vartheta _{1}+ \frac{1-s}{2}\vartheta _{2} \biggr)\,ds\,dt \\& \qquad {}+ \int _{0}^{1} \int _{0}^{1} \biggl[ \frac{1}{\gamma _{1}^{\alpha }}- \biggl( \frac{1-(1-t)^{\gamma _{1}}}{\gamma _{1}} \biggr) ^{\alpha } \biggr] \biggl[ \frac{1}{\gamma _{2}^{\beta }}- \biggl( \frac{1-(1-s)^{\gamma _{2}}}{\gamma _{2}} \biggr) ^{\beta } \biggr] \\& \qquad {} \times \frac {\partial ^{2}\xi }{\partial t\partial s} \biggl( \frac{1-t}{2}\eta _{1}+\frac{1+t}{2}\eta _{2},\frac{1-s}{2}\vartheta _{1}+ \frac{1+s}{2}\vartheta _{2} \biggr)\,ds\,dt \biggr\} , \end{aligned}$$
(2.1)
where
$$\begin{aligned} A =& \frac{2^{\gamma _{2}\beta -1} \gamma _{2}^{\beta } \Gamma (\beta +1)}{ ( \vartheta _{2}-\vartheta _{1} ) ^{\gamma _{2}\beta }} \\ &{}\times \biggl[ ^{\gamma _{2}}I_{\vartheta _{1}^{+}}^{\beta } \xi \biggl( \frac{\eta _{1}+\eta _{2}}{2},\frac{\vartheta _{1}+\vartheta _{2}}{2} \biggr) +{}^{\gamma _{2}}I_{\vartheta _{2}^{-}}^{\beta }\xi \biggl( \frac{\eta _{1}+\eta _{2}}{2},\frac{\vartheta _{1}+\vartheta _{2}}{2} \biggr) \biggr] \\ & {}+. \frac{2^{\gamma _{1}\alpha -1} \gamma _{1}^{\alpha } \Gamma (\alpha +1)}{ ( \eta _{2}-\eta _{1} ) ^{\gamma _{1}\alpha }} \biggl[ ^{ \gamma _{1}}I_{\eta _{1}^{+}}^{\alpha }\xi \biggl( \frac{\eta _{1}+\eta _{2}}{2},\frac{\vartheta _{1}+\vartheta _{2}}{2} \biggr) +{}^{\gamma _{1}}I_{ \eta _{2}^{-}}^{\alpha }\xi \biggl( \frac{\eta _{1}+\eta _{2}}{2},\frac{\vartheta _{1}+\vartheta _{2}}{2} \biggr) \biggr] . \end{aligned}$$
(2.2)
Proof
By integration by parts, we get
$$\begin{aligned} I_{1} =& \int _{0}^{1} \int _{0}^{1} \biggl[ \frac{1}{\gamma _{1}^{\alpha }}- \biggl( \frac{1-(1-t)^{\gamma _{1}}}{\gamma _{1}} \biggr) ^{\alpha } \biggr] \biggl[ \frac{1}{\gamma _{2}^{\beta }}- \biggl( \frac{1-(1-s)^{\gamma _{2}}}{\gamma _{2}} \biggr) ^{\beta } \biggr] \frac {\partial ^{2}\xi }{\partial t\partial s} \\ &{}\times \biggl( \frac{1+t}{2} \eta _{1}+\frac{1-t}{2}\eta _{2},\frac{1+s}{2}\vartheta _{1}+\frac{1-s}{2} \vartheta _{2} \biggr)\,ds\,dt \\ =& \int _{0}^{1} \biggl[ \frac{1}{\gamma _{2}^{\beta }}- \biggl( \frac{1-(1-s)^{\gamma _{2}}}{\gamma _{2}} \biggr) ^{\beta } \biggr] \biggl[ \frac{1}{\gamma _{1}^{\alpha }}- \biggl( \frac{1-(1-t)^{\gamma _{1}}}{\gamma _{1}} \biggr) ^{\alpha } \biggr] \\ &{}\times \biggl\{ \frac{-2}{(\eta _{2}-\eta _{1})} \frac {\partial \xi }{\partial s} \biggl( \frac{1+t}{2}\eta _{1}+ \frac{1-t}{2}\eta _{2}, \frac{1+s}{2}\vartheta _{1}+\frac{1-s}{2}\vartheta _{2} \biggr) \bigg\vert _{0}^{1} \\ & {}+ \int _{0}^{1} \frac{2\alpha }{(\eta _{2}-\eta _{1})} \biggl( \frac{1-(1-t)^{\gamma _{1}}}{\gamma _{1}} \biggr) ^{\alpha -1}(1-t)^{ \gamma _{1}-1}\frac {\partial \xi }{\partial s} \\ & {}\times \biggl( \frac{1+t}{2} \eta _{1}+\frac{1-t}{2}\eta _{2},\frac{1+s}{2}\vartheta _{1}+\frac{1-s}{2} \vartheta _{2} \biggr)\,dt \biggr\} \,ds \\ =& \int _{0}^{1} \biggl[ \frac{1}{\gamma _{2}^{\beta }}- \biggl( \frac{1-(1-s)^{\gamma _{2}}}{\gamma _{2}} \biggr) ^{\beta } \biggr] \biggl\{ \biggl( \frac{2}{\eta _{2}-\eta _{1}} \biggr) \frac{1}{\gamma _{1}^{\alpha }}\frac {\partial \xi }{\partial s} \biggl( \frac{\eta _{1}+\eta _{2}}{2}, \frac{1+s}{2}\vartheta _{1}+ \frac{1-s}{2}\vartheta _{2} \biggr) \\ &{}- \frac{2\alpha }{(\eta _{2}-\eta _{1})} \int _{0}^{1} \biggl( \frac{1-(1-t)^{\gamma _{1}}}{\gamma _{1}} \biggr) ^{\alpha -1}(1-t)^{ \gamma _{1}-1}\frac {\partial \xi }{\partial s} \\ &{}\times \biggl( \frac{1+t}{2} \eta _{1}+\frac{1-t}{2}\eta _{2},\frac{1+s}{2} \vartheta _{1}+\frac{1-s}{2}\vartheta _{2} \biggr)\,dt \biggr\} \,ds \\ =& \frac{2}{ ( \eta _{2}-\eta _{1} ) \gamma _{1}^{\alpha }}\int _{0}^{1} \biggl[ \frac{1}{\gamma _{2}^{\beta }}- \biggl( \frac{1-(1-s)^{\gamma _{2}}}{\gamma _{2}} \biggr) ^{\beta } \biggr] \frac {\partial \xi }{\partial s} \biggl( \frac{\eta _{1}+\eta _{2}}{2},\frac{1+s}{2}\vartheta _{1}+ \frac{1-s}{2} \vartheta _{2} \biggr)\,ds \\ &{}-\frac{2\alpha }{(\eta _{2}-\eta _{1})} \biggl[ \int _{0}^{1} \biggl( \frac{1-(1-t)^{\gamma _{1}}}{\gamma _{1}} \biggr) ^{\alpha -1}(1-t)^{ \gamma _{1}-1} \\ &{}\times \biggl\{ \int _{0}^{1} \biggl[ \frac{1}{\gamma _{2}^{\beta }}- \biggl( \frac{1-(1-s)^{\gamma _{2}}}{\gamma _{2}} \biggr) ^{\beta } \biggr] \\ &{}\times \frac {\partial \xi }{\partial s} \biggl( \frac{1+t}{2}\eta _{1}+ \frac{1-t}{2}\eta _{2}, \frac{1+s}{2}\vartheta _{1}+\frac{1-s}{2}\vartheta _{2} \biggr)\,ds \biggr\} \,dt \biggr] \\ =&\frac{2}{ ( \eta _{2}-\eta _{1} ) } \biggl( \frac{1}{\gamma _{1}} \biggr) ^{\alpha } \biggl[ \biggl( \frac{1}{\gamma _{2}} \biggr) ^{ \beta } \frac{2}{ ( \vartheta _{2}-\vartheta _{1} ) }\xi \biggl( \frac{\eta _{1}+\eta _{2}}{2},\frac{\vartheta _{1}+\vartheta _{2}}{2} \biggr) \\ &{}- \frac{2\beta }{(\vartheta _{2}-\vartheta _{1})} \int _{0}^{1} \biggl( \frac{1-(1-s)^{\gamma _{2}}}{\gamma _{2}} \biggr) ^{\beta -1}(1-s)^{ \gamma _{2}-1}\xi \biggl( \frac{\eta _{1}+\eta _{2}}{2}, \frac{1+s}{2}\vartheta _{1}+\frac{1-s}{2}\vartheta _{2} \biggr)\,ds \biggr] \\ &{}-\frac{2\alpha }{(\eta _{2}-\eta _{1})} \int _{0}^{1} \biggl( \frac{1-(1-t)^{\gamma _{1}}}{\gamma _{1}} \biggr) ^{\alpha -1} \\ &{}\times (1-t)^{ \gamma _{1}-1} \biggl\{ \biggl( \frac{1}{\gamma _{2}} \biggr) ^{\beta } \frac{2}{ ( \vartheta _{2}-\vartheta _{1} ) }\xi \biggl( \frac{1+t}{2}\eta _{1}+\frac{1-t}{2}\eta _{2}, \frac{\vartheta _{1}+\vartheta _{2}}{2} \biggr) \\ &{}- \frac{2\beta }{(\vartheta _{2}-\vartheta _{1})} \int _{0}^{1} \biggl( \frac{1-(1-s)^{\gamma _{2}}}{\gamma _{2}} \biggr) ^{\beta -1} \\ &{}\times (1-s)^{ \gamma _{2}-1}\xi \biggl( \frac{1+t}{2}\eta _{1}+\frac{1-t}{2}\eta _{2}, \frac{1+s}{2} \vartheta _{1}+\frac{1-s}{2}\vartheta _{2} \biggr)\,ds \biggr\} \,dt \\ =&\frac{4}{(\eta _{2}-\eta _{1})(\vartheta _{2}-\vartheta _{1})} \frac{1}{\gamma _{1}^{\alpha }\gamma _{2}^{\beta }}\xi \biggl( \frac{\eta _{1}+\eta _{2}}{2}, \frac{\vartheta _{1}+\vartheta _{2}}{2} \biggr) \\ &{}- \frac{4\beta }{(\eta _{2}-\eta _{1})(\vartheta _{2}-\vartheta _{1})} \biggl( \frac{1}{\gamma _{1}} \biggr) ^{\alpha } \int _{0}^{1} \biggl( \frac{1-(1-s)^{\gamma _{2}}}{\gamma _{2}} \biggr) ^{\beta -1} \\ &{}\times (1-s)^{ \gamma _{2}-1}\xi \biggl( \frac{\eta _{1}+\eta _{2}}{2}, \frac{1+s}{2} \vartheta _{1}+\frac{1-s}{2} \vartheta _{2} \biggr)\,ds \\ &{}- \frac{4\alpha }{(\eta _{2}-\eta _{1})(\vartheta _{2}-\vartheta _{1})} \biggl( \frac{1}{\gamma _{2}} \biggr) ^{\beta } \int _{0}^{1} \biggl( \frac{1-(1-t)^{\gamma _{1}}}{\gamma _{1}} \biggr) ^{\alpha -1} \\ &{}\times (1-t)^{ \gamma _{1}-1}\xi \biggl( \frac{1+t}{2}\eta _{1}+\frac{1-t}{2}\eta _{2}, \frac{\vartheta _{1}+\vartheta _{2}}{2} \biggr)\,dt \\ & {}+ \frac{4\alpha \beta }{(\eta _{2}-\eta _{1})(\vartheta _{2}-\vartheta _{1})} \biggl[ \int _{0}^{1} \int _{0}^{1} \biggl( \frac{1-(1-t)^{\gamma _{1}}}{\gamma _{1}} \biggr) ^{\alpha -1} \\ &{}\times (1-t)^{ \gamma _{1}-1} \biggl( \frac{1-(1-s)^{\gamma _{2}}}{\gamma _{2}} \biggr) ^{\beta -1}(1-s)^{\gamma _{2}-1} \\ &{}\times \xi \biggl( \frac{1+t}{2}\eta _{1}+ \frac{1-t}{2} \eta _{2},\frac{1+s}{2}\vartheta _{1}+\frac{1-s}{2}\vartheta _{2} \biggr)\,ds\,dt \biggr] . \end{aligned}$$
(2.3)
In (2.3), using the change of the variables, we can write
$$\begin{aligned} I_{1} =& \frac{4}{(\eta _{2}-\eta _{1})(\vartheta _{2}-\vartheta _{1})} \frac{1}{\gamma _{1}^{\alpha }\gamma _{2}^{\beta }}\xi \biggl( \frac{\eta _{1}+\eta _{2}}{2},\frac{\vartheta _{1}+\vartheta _{2}}{2} \biggr) \\ &{}- \frac{4\beta }{(\eta _{2}-\eta _{1})(\vartheta _{2}-\vartheta _{1})} \frac{1}{\gamma _{1}^{\alpha }} \biggl( \frac{2}{\vartheta _{2}-\vartheta _{1}} \biggr) ^{\gamma _{2}\beta }\Gamma (\beta ) \bigl( ^{\gamma _{2}}I_{ \vartheta _{1}^{+}}^{\beta } \xi \bigr) \biggl( \frac{\eta _{1}+\eta _{2}}{2},\frac{\vartheta _{1}+\vartheta _{2}}{2} \biggr) \\ &{}- \frac{4\alpha }{(\eta _{2}-\eta _{1})(\vartheta _{2}-\vartheta _{1})}\frac{1}{\gamma _{2}^{\beta }} \biggl( \frac{2}{\eta _{2}-\eta _{1}} \biggr) ^{\gamma _{1}\alpha }\Gamma (\alpha )^{\gamma _{1}}I_{\eta _{1}^{+}}^{ \alpha } \xi \biggl( \frac{\eta _{1}+\eta _{2}}{2}, \frac{\vartheta _{1}+\vartheta _{2}}{2} \biggr) \\ & {}+ \frac{4\alpha \beta }{(\eta _{2}-\eta _{1})(\vartheta _{2}-\vartheta _{1})} \frac{2^{\gamma _{1}\alpha }2^{\gamma _{2}\beta }\Gamma (\alpha )\Gamma (\beta )}{ ( \eta _{2}-\eta _{1} ) ^{\gamma _{1}\alpha } ( \vartheta _{2}-\vartheta _{1} ) ^{\gamma _{2}\beta }} \bigl( ^{ \gamma _{1}\gamma _{2}}I_{\eta _{1}^{+},\vartheta _{1}^{+}}^{\alpha , \beta } \xi \bigr) \\ &{}\times \biggl( \frac{\eta _{1}+\eta _{2}}{2}, \frac{\vartheta _{1}+\vartheta _{2}}{2} \biggr) . \end{aligned}$$
(2.4)
Thus, similarly, by integration by parts it follows that
$$\begin{aligned}& I_{2} = \int _{0}^{1} \int _{0}^{1} \biggl[ \frac{1}{\gamma _{1}^{\alpha }}- \biggl( \frac{1-(1-t)^{\gamma _{1}}}{\gamma _{1}} \biggr) ^{\alpha } \biggr] \biggl[ \frac{1}{\gamma _{2}^{\beta }}- \biggl( \frac{1-(1-s)^{\gamma _{2}}}{\gamma _{2}} \biggr) ^{\beta } \biggr] \\& \hphantom{I_{2} =} {}\times \frac {\partial ^{2}\xi }{\partial t\partial s} \biggl( \frac{1+t}{2} \eta _{1}+\frac{1-t}{2}\eta _{2},\frac{1-s}{2}\vartheta _{1}+\frac{1+s}{2} \vartheta _{2} \biggr)\,ds\,dt \\& \hphantom{I_{2} } = \frac{-4}{(\eta _{2}-\eta _{1})(\vartheta _{2}-\vartheta _{1})} \frac{1}{\gamma _{1}^{\alpha }\gamma _{2}^{\beta }}\xi \biggl( \frac{\eta _{1}+\eta _{2}}{2}, \frac{\vartheta _{1}+\vartheta _{2}}{2} \biggr) \\& \hphantom{I_{2} =} {}+ \frac{4\beta }{(\eta _{2}-\eta _{1})(\vartheta _{2}-\vartheta _{1})} \frac{1}{\gamma _{1}^{\alpha }} \biggl( \frac{2}{\vartheta _{2}-\vartheta _{1}} \biggr) ^{\gamma _{2}\beta }\Gamma (\beta ) \bigl( ^{\gamma _{2}}I_{ \vartheta _{2}^{-}}^{\beta } \xi \bigr) \biggl( \frac{\eta _{1}+\eta _{2}}{2},\frac{\vartheta _{1}+\vartheta _{2}}{2} \biggr) \\& \hphantom{I_{2} =} {}+ \frac{4\alpha }{(\eta _{2}-\eta _{1})(\vartheta _{2}-\vartheta _{1})}\frac{1}{\gamma _{2}^{\beta }} \biggl( \frac{2}{\eta _{2}-\eta _{1}} \biggr) ^{\gamma _{1}\alpha }\Gamma (\alpha ) \bigl( ^{\gamma _{1}}I_{ \eta _{1}^{+}}^{\alpha } \xi \bigr) \biggl( \frac{\eta _{1}+\eta _{2}}{2}, \frac{\vartheta _{1}+\vartheta _{2}}{2} \biggr) \\& \hphantom{I_{2} =} {}- \frac{4\alpha \beta }{(\eta _{2}-\eta _{1})(\vartheta _{2}-\vartheta _{1})} \frac{2^{\gamma _{1}\alpha }2^{\gamma _{2}\beta }\Gamma (\alpha )\Gamma (\beta )}{ ( \eta _{2}-\eta _{1} ) ^{\gamma _{1}\alpha } ( \vartheta _{2}-\vartheta _{1} ) ^{\gamma _{2}\beta }} \bigl( ^{ \gamma _{1}\gamma _{2}}I_{\eta _{1}^{+},\vartheta _{2}^{-}}^{\alpha , \beta } \xi \bigr) \\& \hphantom{I_{2} =} {}\times \biggl( \frac{\eta _{1}+\eta _{2}}{2}, \frac{\vartheta _{1}+\vartheta _{2}}{2} \biggr) , \end{aligned}$$
(2.5)
$$\begin{aligned}& I_{3} = \int _{0}^{1} \int _{0}^{1} \biggl[ \frac{1}{\gamma _{1}^{\alpha }}- \biggl( \frac{1-(1-t)^{\gamma _{1}}}{\gamma _{1}} \biggr) ^{\alpha } \biggr] \biggl[ \frac{1}{\gamma _{2}^{\beta }}- \biggl( \frac{1-(1-s)^{\gamma _{2}}}{\gamma _{2}} \biggr) ^{\beta } \biggr] \\& \hphantom{I_{3} =} {}\times \frac {\partial ^{2}\xi }{\partial t\partial s} \biggl( \frac{1-t}{2} \eta _{1}+\frac{1+t}{2}\eta _{2},\frac{1+s}{2}\vartheta _{1}+\frac{1-s}{2} \vartheta _{2} \biggr)\,ds\,dt \\& \hphantom{I_{3}} = \frac{-4}{(\eta _{2}-\eta _{1})(\vartheta _{2}-\vartheta _{1})} \frac{1}{\gamma _{1}^{\alpha }\gamma _{2}^{\beta }}\xi \biggl( \frac{\eta _{1}+\eta _{2}}{2}, \frac{\vartheta _{1}+\vartheta _{2}}{2} \biggr) \\& \hphantom{I_{3} =} {}+ \frac{4\beta }{(\eta _{2}-\eta _{1})(\vartheta _{2}-\vartheta _{1})} \frac{1}{\gamma _{1}^{\alpha }} \biggl( \frac{2}{\vartheta _{2}-\vartheta _{1}} \biggr) ^{\gamma _{2}\beta }\Gamma (\beta ) \bigl( ^{\gamma _{2}}I_{ \vartheta _{1}^{+}}^{\beta } \xi \bigr) \biggl( \frac{\eta _{1}+\eta _{2}}{2},\frac{\vartheta _{1}+\vartheta _{2}}{2} \biggr) \\& \hphantom{I_{3} =} {}+ \frac{4\alpha }{(\eta _{2}-\eta _{1})(\vartheta _{2}-\vartheta _{1})}\frac{1}{\gamma _{2}^{\beta }} \biggl( \frac{2}{\eta _{2}-\eta _{1}} \biggr) ^{\gamma _{1}\alpha }\Gamma (\alpha ) \bigl( ^{\gamma _{1}}I_{ \eta _{2}^{-}}^{\alpha } \xi \bigr) \biggl( \frac{\eta _{1}+\eta _{2}}{2}, \frac{\vartheta _{1}+\vartheta _{2}}{2} \biggr) \\& \hphantom{I_{3} =} {}- \frac{4\alpha \beta }{(\eta _{2}-\eta _{1})(\vartheta _{2}-\vartheta _{1})} \frac{2^{\gamma _{1}\alpha }2^{\gamma _{2}\beta }\Gamma (\alpha )\Gamma (\beta )}{ ( \eta _{2}-\eta _{1} ) ^{\gamma _{1}\alpha } ( \vartheta _{2}-\vartheta _{1} ) ^{\gamma _{2}\beta }} \bigl( ^{ \gamma _{1}\gamma _{2}}I_{\eta _{2}^{-},\vartheta _{1}^{+}}^{\alpha , \beta } \xi \bigr) \\& \hphantom{I_{3} =} {}\times \biggl( \frac{\eta _{1}+\eta _{2}}{2}, \frac{\vartheta _{1}+\vartheta _{2}}{2} \biggr) , \end{aligned}$$
(2.6)
and
$$\begin{aligned} I_{4} =& \int _{0}^{1} \int _{0}^{1} \biggl[ \frac{1}{\gamma _{1}^{\alpha }}- \biggl( \frac{1-(1-t)^{\gamma _{1}}}{\gamma _{1}} \biggr) ^{\alpha } \biggr] \biggl[ \frac{1}{\gamma _{2}^{\beta }}- \biggl( \frac{1-(1-s)^{\gamma _{2}}}{\gamma _{2}} \biggr) ^{\beta } \biggr] \\ &{}\times \frac {\partial ^{2}\xi }{\partial t\partial s} \biggl( \frac{1-t}{2} \eta _{1}+\frac{1+t}{2}\eta _{2},\frac{1-s}{2}\vartheta _{1}+\frac{1+s}{2} \vartheta _{2} \biggr)\,ds\,dt \\ =&\frac{4}{(\eta _{2}-\eta _{1})(\vartheta _{2}-\vartheta _{1})} \frac{1}{\gamma _{1}^{\alpha }\gamma _{2}^{\beta }}\xi \biggl( \frac{\eta _{1}+\eta _{2}}{2}, \frac{\vartheta _{1}+\vartheta _{2}}{2} \biggr) \\ &{}- \frac{4\beta }{(\eta _{2}-\eta _{1})(\vartheta _{2}-\vartheta _{1})} \frac{1}{\gamma _{1}^{\alpha }} \biggl( \frac{2}{\vartheta _{2}-\vartheta _{1}} \biggr) ^{\gamma _{2}\beta }\Gamma (\beta ) \bigl( ^{\gamma _{2}}I_{ \vartheta _{2}^{-}}^{\beta } \xi \bigr) \biggl( \frac{\eta _{1}+\eta _{2}}{2},\frac{\vartheta _{1}+\vartheta _{2}}{2} \biggr) \\ &{}- \frac{4\alpha }{(\eta _{2}-\eta _{1})(\vartheta _{2}-\vartheta _{1})}\frac{1}{\gamma _{2}^{\beta }} \biggl( \frac{2}{\eta _{2}-\eta _{1}} \biggr) ^{\gamma _{1}\alpha }\Gamma (\alpha ) \bigl( ^{\gamma _{1}}I_{ \eta _{2}^{-}}^{\alpha } \xi \bigr) \biggl( \frac{\eta _{1}+\eta _{2}}{2}, \frac{\vartheta _{1}+\vartheta _{2}}{2} \biggr) \\ & {}+ \frac{4\alpha \beta }{(\eta _{2}-\eta _{1})(\vartheta _{2}-\vartheta _{1})} \frac{2^{\gamma _{1}\alpha }2^{\gamma _{2}\beta }\Gamma (\alpha )\Gamma (\beta )}{ ( \eta _{2}-\eta _{1} ) ^{\gamma _{1}\alpha } ( \vartheta _{2}-\vartheta _{1} ) ^{\gamma _{2}\beta }} \bigl( ^{ \gamma _{1}\gamma _{2}}I_{\eta _{2}^{-},\vartheta _{2}^{-}}^{\alpha , \beta } \xi \bigr) \\ &{}\times \biggl( \frac{\eta _{1}+\eta _{2}}{2}, \frac{\vartheta _{1}+\vartheta _{2}}{2} \biggr) . \end{aligned}$$
(2.7)
By the equalities (2.4)–(2.7), we obtain
$$\begin{aligned}& \frac{\gamma _{1}^{\alpha }\gamma _{2}^{\beta } ( \eta _{2}-\eta _{1} ) ( \vartheta _{2}-\vartheta _{1} ) }{16} [ I_{1}-I_{2}-I_{3}+I_{4} ] \\& \quad = \xi \biggl( \frac{\eta _{1}+\eta _{2}}{2}, \frac{\vartheta _{1}+\vartheta _{2}}{2} \biggr) + \frac{2^{\gamma _{1}\alpha -1}2^{\gamma _{2}\beta -1}\Gamma (\alpha +1)\Gamma (\beta +1)\gamma _{1}^{\alpha }\gamma _{2}^{\beta }}{ ( \eta _{2}-\eta _{1} ) ^{\gamma _{1}\alpha } ( \vartheta _{2}-\vartheta _{1} ) ^{\gamma _{2}\beta }} \\& \qquad {}\times \biggl[ ^{\gamma _{1}\gamma _{2}}I_{\eta _{1}^{+},\vartheta _{1}^{+}}^{ \alpha ,\beta }f \biggl( \frac{\eta _{1}+\eta _{2}}{2}, \frac{\vartheta _{1}+\vartheta _{2}}{2} \biggr) +^{\gamma _{1}\gamma _{2}}I_{ \eta _{1}^{+},\vartheta _{2}^{-}}^{\alpha ,\beta }f \biggl( \frac{\eta _{1} +\eta _{2}}{2},\frac{\vartheta _{1}+\vartheta _{2}}{2} \biggr) \\& \qquad {} +{}^{\gamma _{1}\gamma _{2}}I_{\eta _{2}^{-},\vartheta _{1}^{+}}^{ \alpha ,\beta }f \biggl( \frac{\eta _{1}+\eta _{2}}{2}, \frac{\vartheta _{1}+\vartheta _{2}}{2} \biggr) \\& \qquad {} +{}^{\gamma _{1}\gamma _{2}}I_{\eta _{2}^{-}, \vartheta _{2}^{-}}^{\alpha ,\beta }\xi \biggl( \frac{\eta _{1}+\eta _{2}}{2}, \frac{\vartheta _{1}+\vartheta _{2}}{2} \biggr) \biggr] -A. \end{aligned}$$
This completes the proof. □
Next, we start to state the first theorem containing the midpoint type inequality for generalized conformable fractional integrals.
Theorem 3
Assume that the assumptions of Lemma 1hold. If \(\vert \frac {\partial ^{2}\xi (t,s)}{\partial t\partial s} \vert \) is a co-ordinated convex function on Δ, then the following inequality holds.
$$\begin{aligned}& \biggl\vert \xi \biggl( \frac{\eta _{1}+\eta _{2}}{2}, \frac{\vartheta _{1}+\vartheta _{2}}{2} \biggr) + \frac{2^{\gamma _{1}\alpha -1}2^{\gamma _{2}\beta -1}\Gamma (\alpha +1)\Gamma (\beta +1)\gamma _{1}^{\alpha }\gamma _{2}^{\beta }}{ ( \eta _{2}-\eta _{1} ) ^{\gamma _{1}\alpha } ( \vartheta _{2}-\vartheta _{1} ) ^{\gamma _{2}\beta }} \\& \qquad {}\times \biggl[ ^{ \gamma _{1}\gamma _{2}}I_{\eta _{1}^{+},\vartheta _{1}^{+}}^{\alpha , \beta }\xi \biggl( \frac{\eta _{1}+\eta _{2}}{2}, \frac{\vartheta _{1}+\vartheta _{2}}{2} \biggr) \\& \qquad {}+ ^{\gamma _{1}\gamma _{2}}I_{\eta _{1}^{+}, \vartheta _{2}^{-}}^{\alpha ,\beta }\xi \biggl( \frac{\eta _{1}+\eta _{2}}{2}, \frac{\vartheta _{1}+\vartheta _{2}}{2} \biggr) +{}^{\gamma _{1} \gamma _{2}}I_{\eta _{2}^{-},\vartheta _{1}^{+}}^{\alpha ,\beta } \xi \biggl( \frac{\eta _{1}+\eta _{2}}{2},\frac{\vartheta _{1}+\vartheta _{2}}{2} \biggr) \\& \qquad {}+{}^{\gamma _{1}\gamma _{2}}I_{\eta _{2}^{-},\vartheta _{2}^{-}}^{ \alpha ,\beta }\xi \biggl( \frac{\eta _{1}+\eta _{2}}{2}, \frac{\vartheta _{1}+\vartheta _{2}}{2} \biggr) \biggr] -A \biggr\vert \\& \quad \leq \frac{ ( \eta _{2}-\eta _{1} ) ( \vartheta _{2}-\vartheta _{1} ) }{16 } \biggl[ 1-\frac{1}{\gamma _{1}}B \biggl( \alpha +1, \frac{1}{\gamma _{1}} \biggr) \biggr] \biggl[ 1- \frac{1}{\gamma _{2}}B \biggl( \beta +1,\frac{1}{\gamma _{2}} \biggr) \biggr] \\& \qquad {}\times \biggl[ \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s}(\eta _{1},\vartheta _{1}) \biggr\vert + \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s}(\eta _{1},\vartheta _{2}) \biggr\vert + \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s}(\eta _{2},\vartheta _{1}) \biggr\vert + \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s}(\eta _{2},\vartheta _{2}) \biggr\vert \biggr] , \end{aligned}$$
(2.8)
where A is defined by (2.2) and \(B ( \cdot ,\cdot ) \) refers to the Beta function.
Proof
From Lemma 1, we acquire
$$\begin{aligned}& \biggl\vert \xi \biggl( \frac{\eta _{1}+\eta _{2}}{2}, \frac{\vartheta _{1}+\vartheta _{2}}{2} \biggr) + \frac{2^{\gamma _{1}\alpha -1}2^{\gamma _{2}\beta -1}\Gamma (\alpha +1)\Gamma (\beta +1)\gamma _{1}^{\alpha }\gamma _{2}^{\beta }}{ ( \eta _{2}-\eta _{1} ) ^{\gamma _{1}\alpha } ( \vartheta _{2}-\vartheta _{1} ) ^{\gamma _{2}\beta }} \\& \qquad {}\times \biggl[ ^{\gamma _{1}\gamma _{2}}I_{\eta _{1}^{+},\vartheta _{1}^{+}}^{ \alpha ,\beta }\xi \biggl( \frac{\eta _{1}+\eta _{2}}{2}, \frac{\vartheta _{1}+\vartheta _{2}}{2} \biggr) +^{\gamma _{1}\gamma _{2}}I_{ \eta _{1}^{+},\vartheta _{2}^{-}}^{\alpha ,\beta } \xi \biggl( \frac{\eta _{1}+\eta _{2}}{2},\frac{\vartheta _{1}+\vartheta _{2}}{2} \biggr) \\& \qquad {}+ {}^{\gamma _{1}\gamma _{2}}I_{\eta _{2}^{-}, \vartheta _{1}^{+}}^{\alpha ,\beta }\xi \biggl( \frac{\eta _{1}+\eta _{2}}{2}, \frac{\vartheta _{1}+\vartheta _{2}}{2} \biggr) {}^{\gamma _{1}\gamma _{2}}I_{ \eta _{2}^{-},\vartheta _{2}^{-}}^{\alpha ,\beta }\xi \biggl( \frac{\eta _{1}+\eta _{2}}{2}, \frac{\vartheta _{1}+\vartheta _{2}}{2} \biggr) \biggr] -A \biggr\vert \\& \quad \leq \frac{\gamma _{1}^{\alpha }\gamma _{2}^{\beta } ( \eta _{2}-\eta _{1} ) ( \vartheta _{2}-\vartheta _{1} ) }{16} \\& \qquad {}\times \biggl\{ \int _{0}^{1} \int _{0}^{1} \biggl[ \frac{1}{\gamma _{1}^{\alpha }}- \biggl( \frac{1-(1-t)^{\gamma _{1}}}{\gamma _{1}} \biggr) ^{\alpha } \biggr] \biggl[ \frac{1}{\gamma _{2}^{\beta }}- \biggl( \frac{1-(1-s)^{\gamma _{2}}}{\gamma _{2}} \biggr) ^{\beta } \biggr] \\& \qquad {}\times \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} \biggl( \frac{1+t}{2}\eta _{1}+\frac{1-t}{2}\eta _{2}, \frac{1+s}{2} \vartheta _{1}+\frac{1-s}{2}\vartheta _{2} \biggr) \biggr\vert \,ds\,dt \\& \qquad {}+ \int _{0}^{1} \int _{0}^{1} \biggl[ \frac{1}{\gamma _{1}^{\alpha }}- \biggl( \frac{1-(1-t)^{\gamma _{1}}}{\gamma _{1}} \biggr) ^{\alpha } \biggr] \biggl[ \frac{1}{\gamma _{2}^{\beta }}- \biggl( \frac{1-(1-s)^{\gamma _{2}}}{\gamma _{2}} \biggr) ^{\beta } \biggr] \\& \qquad {}\times \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} \biggl( \frac{1+t}{2}\eta _{1}+\frac{1-t}{2}\eta _{2}, \frac{1-s}{2} \vartheta _{1}+\frac{1+s}{2}\vartheta _{2} \biggr) \biggr\vert \,ds\,dt \\& \qquad {}+ \int _{0}^{1} \int _{0}^{1} \biggl[ \frac{1}{\gamma _{1}^{\alpha }}- \biggl( \frac{1-(1-t)^{\gamma _{1}}}{\gamma _{1}} \biggr) ^{\alpha } \biggr] \biggl[ \frac{1}{\gamma _{2}^{\beta }}- \biggl( \frac{1-(1-s)^{\gamma _{2}}}{\gamma _{2}} \biggr) ^{\beta } \biggr] \\& \qquad {}\times \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} \biggl( \frac{1-t}{2}\eta _{1}+\frac{1+t}{2}\eta _{2}, \frac{1+s}{2} \vartheta _{1}+\frac{1-s}{2}\vartheta _{2} \biggr) \biggr\vert \,ds\,dt \\& \qquad {}+ \int _{0}^{1} \int _{0}^{1} \biggl[ \frac{1}{\gamma _{1}^{\alpha }}- \biggl( \frac{1-(1-t)^{\gamma _{1}}}{\gamma _{1}} \biggr) ^{\alpha } \biggr] \biggl[ \frac{1}{\gamma _{2}^{\beta }}- \biggl( \frac{1-(1-s)^{\gamma _{2}}}{\gamma _{2}} \biggr) ^{\beta } \biggr] \\& \qquad {}\times \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} \biggl( \frac{1-t}{2}\eta _{1}+\frac{1+t}{2}\eta _{2}, \frac{1-s}{2} \vartheta _{1}+\frac{1+s}{2}\vartheta _{2} \biggr) \biggr\vert \,ds\,dt \biggr\} . \end{aligned}$$
(2.9)
Since \(\vert \frac {\partial ^{2}\xi }{\partial t\partial s} \vert \) is co-ordinated convex function on Δ, then one has:
$$\begin{aligned}& \biggl\vert \xi \biggl( \frac{\eta _{1}+\eta _{2}}{2}, \frac{\vartheta _{1}+\vartheta _{2}}{2} \biggr) + \frac{2^{\gamma _{1}\alpha -1}2^{\gamma _{2}\beta -1}\Gamma (\alpha +1)\Gamma (\beta +1)\gamma _{1}^{\alpha }\gamma _{2}^{\beta }}{ ( \eta _{2}-\eta _{1} ) ^{\gamma _{1}\alpha } ( \vartheta _{2}-\vartheta _{1} ) ^{\gamma _{2}\beta }} \\& \qquad {}\times \biggl[ ^{ \gamma _{1}\gamma _{2}}I_{\eta _{1}^{+},\vartheta _{1}^{+}}^{\alpha , \beta }\xi \biggl( \frac{\eta _{1}+\eta _{2}}{2}, \frac{\vartheta _{1}+\vartheta _{2}}{2} \biggr) \\& \qquad {}+ ^{\gamma _{1}\gamma _{2}}I_{\eta _{1}^{+}, \vartheta _{2}^{-}}^{\alpha ,\beta }\xi \biggl( \frac{\eta _{1}+\eta _{2}}{2}, \frac{\vartheta _{1}+\vartheta _{2}}{2} \biggr) +{}^{\gamma _{1} \gamma _{2}}I_{\eta _{2}^{-},\vartheta _{1}^{+}}^{\alpha ,\beta } \xi \biggl( \frac{\eta _{1}+\eta _{2}}{2},\frac{\vartheta _{1}+\vartheta _{2}}{2} \biggr) \\& \qquad {} +{}^{\gamma _{1}\gamma _{2}}I_{\eta _{2}^{-},\vartheta _{2}^{-}}^{ \alpha ,\beta }\xi \biggl( \frac{\eta _{1}+\eta _{2}}{2}, \frac{\vartheta _{1}+\vartheta _{2}}{2} \biggr) \biggr] -A \biggr\vert \\& \quad \leq \frac{\gamma _{1}^{\alpha }\gamma _{2}^{\beta } ( \eta _{2}-\eta _{1} ) ( \vartheta _{2}-\vartheta _{1} ) }{16} \\& \qquad {}\times \biggl\{ \int _{0}^{1} \int _{0}^{1} \biggl[ \frac{1}{\gamma _{1}^{\alpha }}- \biggl( \frac{1-(1-t)^{\gamma _{1}}}{\gamma _{1}} \biggr) ^{\alpha } \biggr] \biggl[ \frac{1}{\gamma _{2}^{\beta }}- \biggl( \frac{1-(1-s)^{\gamma _{2}}}{\gamma _{2}} \biggr) ^{\beta } \biggr] \\& \qquad {}\times \biggl[ \biggl( \frac{1+t}{2} \biggr) \biggl( \frac{1+s}{2} \biggr) \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} ( \eta _{1},\vartheta _{1} ) \biggr\vert + \biggl( \frac{1+t}{2} \biggr) \biggl( \frac{1-s}{2} \biggr) \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} ( \eta _{1}, \vartheta _{2} ) \biggr\vert \\& \qquad {}+ \biggl( \frac{1-t}{2} \biggr) \biggl( \frac{1+s}{2} \biggr) \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} ( \eta _{2},\vartheta _{1} ) \biggr\vert + \biggl( \frac{1-t}{2} \biggr) \biggl( \frac{1-s}{2} \biggr) \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} ( \eta _{2}, \vartheta _{2} ) \biggr\vert \biggr]\,ds\,dt \\& \qquad {}+ \int _{0}^{1} \int _{0}^{1} \biggl[ \frac{1}{\gamma _{1}^{\alpha }}- \biggl( \frac{1-(1-t)^{\gamma _{1}}}{\gamma _{1}} \biggr) ^{\alpha } \biggr] \biggl[ \frac{1}{\gamma _{2}^{\beta }}- \biggl( \frac{1-(1-s)^{\gamma _{2}}}{\gamma _{2}} \biggr) ^{\beta } \biggr] \\& \qquad {}\times \biggl[ \biggl( \frac{1+t}{2} \biggr) \biggl( \frac{1-s}{2} \biggr) \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} ( \eta _{1},\vartheta _{1} ) \biggr\vert + \biggl( \frac{1+t}{2} \biggr) \biggl( \frac{1+s}{2} \biggr) \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} ( \eta _{1}, \vartheta _{2} ) \biggr\vert \\& \qquad {}+ \biggl( \frac{1-t}{2} \biggr) \biggl( \frac{1-s}{2} \biggr) \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} ( \eta _{2},\vartheta _{1} ) \biggr\vert + \biggl( \frac{1-t}{2} \biggr) \biggl( \frac{1+s}{2} \biggr) \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} ( \eta _{2}, \vartheta _{2} ) \biggr\vert \biggr]\,ds\,dt \\& \qquad {}+ \int _{0}^{1} \int _{0}^{1} \biggl[ \frac{1}{\gamma _{1}^{\alpha }}- \biggl( \frac{1-(1-t)^{\gamma _{1}}}{\gamma _{1}} \biggr) ^{\alpha } \biggr] \biggl[ \frac{1}{\gamma _{2}^{\beta }}- \biggl( \frac{1-(1-s)^{\gamma _{2}}}{\gamma _{2}} \biggr) ^{\beta } \biggr] \\& \qquad {}\times \biggl[ \biggl( \frac{1-t}{2} \biggr) \biggl( \frac{1+s}{2} \biggr) \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} ( \eta _{1},\vartheta _{1} ) \biggr\vert + \biggl( \frac{1-t}{2} \biggr) \biggl( \frac{1-s}{2} \biggr) \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} ( \eta _{1}, \vartheta _{2} ) \biggr\vert \\& \qquad {}+ \biggl( \frac{1+t}{2} \biggr) \biggl( \frac{1+s}{2} \biggr) \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} ( \eta _{2},\vartheta _{1} ) \biggr\vert + \biggl( \frac{1+t}{2} \biggr) \biggl( \frac{1-s}{2} \biggr) \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} ( \eta _{2}, \vartheta _{2} ) \biggr\vert \biggr]\,ds\,dt \\& \qquad {}+ \int _{0}^{1} \int _{0}^{1} \biggl[ \frac{1}{\gamma _{1}^{\alpha }}- \biggl( \frac{1-(1-t)^{\gamma _{1}}}{\gamma _{1}} \biggr) ^{\alpha } \biggr] \biggl[ \frac{1}{\gamma _{2}^{\beta }}- \biggl( \frac{1-(1-s)^{\gamma _{2}}}{\gamma _{2}} \biggr) ^{\beta } \biggr] \\& \qquad {}\times \biggl[ \biggl( \frac{1-t}{2} \biggr) \biggl( \frac{1-s}{2} \biggr) \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} ( \eta _{1},\vartheta _{1} ) \biggr\vert + \biggl( \frac{1-t}{2} \biggr) \biggl( \frac{1+s}{2} \biggr) \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} ( \eta _{1}, \vartheta _{2} ) \biggr\vert \\& \qquad {}+ \biggl( \frac{1+t}{2} \biggr) \biggl( \frac{1-s}{2} \biggr) \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} ( \eta _{2}, \vartheta _{1} ) \biggr\vert + \biggl( \frac{1+t}{2} \biggr) \biggl( \frac{1+s}{2} \biggr) \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} ( \eta _{2}, \vartheta _{2} ) \biggr\vert \biggr]\,ds\,dt \biggr\} \\& \quad = \frac{\gamma _{1}^{\alpha }\gamma _{2}^{\beta } ( \eta _{2}-\eta _{1} ) ( \vartheta _{2}-\vartheta _{1} ) }{16} \\& \qquad {}\times \biggl( \int _{0}^{1} \int _{0}^{1} \biggl[ \frac{1}{\gamma _{1}^{\alpha }}- \biggl( \frac{1-(1-t)^{\gamma _{1}}}{\gamma _{1}} \biggr) ^{\alpha } \biggr] \biggl[ \frac{1}{\gamma _{2}^{\beta }}- \biggl( \frac{1-(1-s)^{\gamma _{2}}}{\gamma _{2}} \biggr) ^{\beta } \biggr] \biggr) \\& \qquad {}\times \biggl[ \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} ( \eta _{1},\vartheta _{1} ) \biggr\vert + \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} ( \eta _{1},\vartheta _{2} ) \biggr\vert + \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} ( \eta _{2}, \vartheta _{1} ) \biggr\vert + \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} ( \eta _{2}, \vartheta _{2} ) \biggr\vert \biggr] \\& \quad = \frac{ ( \eta _{2}-\eta _{1} ) ( \vartheta _{2}-\vartheta _{1} ) }{16 } \biggl[ 1-\frac{1}{\gamma _{1}}B \biggl( \alpha +1, \frac{1}{\gamma _{1}} \biggr) \biggr] \biggl[ 1-\frac{1}{\gamma _{2}}B \biggl( \beta +1,\frac{1}{\gamma _{2}} \biggr) \biggr] \\& \qquad {}\times \biggl[ \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} ( \eta _{1},\vartheta _{1} ) \biggr\vert + \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} ( \eta _{1},\vartheta _{2} ) \biggr\vert + \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} ( \eta _{2}, \vartheta _{1} ) \biggr\vert + \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} ( \eta _{2}, \vartheta _{2} ) \biggr\vert \biggr] , \end{aligned}$$
which finishes the proof. □
Remark 3
In Theorem 3, if we choose \(\gamma _{1}=1\) and \(\gamma _{2}=1\), then the following inequality for Riemann–Liouville fractional integrals is achieved
$$\begin{aligned}& \biggl\vert \xi \biggl( \frac{\eta _{1}+\eta _{2}}{2}, \frac{\vartheta _{1}+\vartheta _{2}}{2} \biggr) + \frac{2^{\alpha -1}2^{\beta -1}\Gamma (\alpha +1)\Gamma (\beta +1)}{ ( \eta _{2}-\eta _{1} ) ^{\alpha } ( \vartheta _{2}-\vartheta _{1} ) ^{\beta }} \\& \qquad {}\times \biggl[ J_{\eta _{1}^{+},\vartheta _{1}^{+}}^{\alpha ,\beta } \xi \biggl( \frac{\eta _{1}+\eta _{2}}{2}, \frac{\vartheta _{1}+\vartheta _{2}}{2} \biggr) \\& \qquad {}+ J_{\eta _{1}^{+},\vartheta _{2}^{-}}^{\alpha , \beta }\xi \biggl( \frac{\eta _{1}+\eta _{2}}{2}, \frac{\vartheta _{1}+\vartheta _{2}}{2} \biggr) +J_{\eta _{2}^{-},\vartheta _{1}^{+}}^{\alpha ,\beta } \xi \biggl( \frac{\eta _{1}+\eta _{2}}{2}, \frac{\vartheta _{1}+\vartheta _{2}}{2} \biggr) \\& \qquad {} +J_{\eta _{2}^{-}, \vartheta _{2}^{-}}^{\alpha ,\beta } \xi \biggl( \frac{\eta _{1}+\eta _{2}}{2},\frac{\vartheta _{1}+\vartheta _{2}}{2} \biggr) \biggr] -D \biggr\vert \\& \quad \leq \frac{ ( \eta _{2}-\eta _{1} ) ( \vartheta _{2}-\vartheta _{1} ) }{16 } \biggl( \frac{\alpha }{\alpha +1} \biggr) \biggl( \frac{\beta }{\beta +1} \biggr) \\& \qquad {}\times \biggl[ \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} ( \eta _{1},\vartheta _{1} ) \biggr\vert + \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} ( \eta _{1},\vartheta _{2} ) \biggr\vert + \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} ( \eta _{2}, \vartheta _{1} ) \biggr\vert + \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} ( \eta _{2}, \vartheta _{2} ) \biggr\vert \biggr] , \end{aligned}$$
(2.10)
where
$$\begin{aligned} D =& \frac{2^{\beta -1}\Gamma (\beta +1)}{ ( \vartheta _{2}-\vartheta _{1} ) ^{\gamma _{2}\beta }} \biggl[ J_{\vartheta _{1}^{+}}^{\beta }\xi \biggl( \frac{\eta _{1}+\eta _{2}}{2}, \frac{\vartheta _{1}+\vartheta _{2}}{2} \biggr) +J_{\vartheta _{2}^{-}}^{\beta } \xi \biggl( \frac{\eta _{1}+\eta _{2}}{2},\frac{\vartheta _{1}+\vartheta _{2}}{2} \biggr) \biggr] \\ & {}+. \frac{2^{\alpha -1}\Gamma (\alpha +1)}{ ( \eta _{2}-\eta _{1} ) ^{\gamma _{1}\alpha }} \biggl[ J_{\eta _{1}^{+}}^{\alpha }\xi \biggl( \frac{\eta _{1}+\eta _{2}}{2},\frac{\vartheta _{1}+\vartheta _{2}}{2} \biggr) +J_{\eta _{2}^{-}}^{\alpha }\xi \biggl( \frac{\eta _{1}+\eta _{2}}{2}, \frac{\vartheta _{1}+\vartheta _{2}}{2} \biggr) \biggr] . \end{aligned}$$
(2.11)
The inequality (2.10) is the same of [10, Remark 5].
Remark 4
If we choose \(\gamma _{1}=\gamma _{2}=\alpha =\beta =1\) in Theorem 3, then Theorem 3 reduces to [23, Theorem 2].
Theorem 4
Assume that the assumptions of Lemma 1hold. If \(\vert \frac {\partial ^{2}\xi }{\partial t\partial s} \vert ^{q} \), \(q>1\), is a co-ordinated convex function on Δ, then the following inequality holds.
$$\begin{aligned}& \biggl\vert \xi \biggl( \frac{\eta _{1}+\eta _{2}}{2}, \frac{\vartheta _{1}+\vartheta _{2}}{2} \biggr) \\& \qquad {} + \frac{2^{\gamma _{1}\alpha -1}2^{\gamma _{2}\beta -1}\Gamma (\alpha +1)\Gamma (\beta +1)\gamma _{1}^{\alpha }\gamma _{2}^{\beta }}{ ( \eta _{2}-\eta _{1} ) ^{\gamma _{1}\alpha } ( \vartheta _{2}-\vartheta _{1} ) ^{\gamma _{2}\beta }} \biggl[ ^{ \gamma _{1}\gamma _{2}}I_{\eta _{1}^{+},\vartheta _{1}^{+}}^{\alpha , \beta }\xi \biggl( \frac{\eta _{1}+\eta _{2}}{2}, \frac{\vartheta _{1}+\vartheta _{2}}{2} \biggr) \\& \qquad {}+ ^{\gamma _{1}\gamma _{2}}I_{\eta _{1}^{+}, \vartheta _{2}^{-}}^{\alpha ,\beta }\xi \biggl( \frac{\eta _{1}+\eta _{2}}{2}, \frac{\vartheta _{1}+\vartheta _{2}}{2} \biggr) +{}^{\gamma _{1} \gamma _{2}}I_{\eta _{2}^{-},\vartheta _{1}^{+}}^{\alpha ,\beta } \xi \biggl( \frac{\eta _{1}+\eta _{2}}{2},\frac{\vartheta _{1}+\vartheta _{2}}{2} \biggr) \\& \qquad {}+{}^{\gamma _{1}\gamma _{2}}I_{\eta _{2}^{-},\vartheta _{2}^{-}}^{ \alpha ,\beta }\xi \biggl( \frac{\eta _{1}+\eta _{2}}{2}, \frac{\vartheta _{1}+\vartheta _{2}}{2} \biggr) \biggr] -A \biggr\vert \\& \quad \leq \frac{ ( \eta _{2}-\eta _{1} ) ( \vartheta _{2}-\vartheta _{1} ) }{16 } \biggl[ \biggl( 16-\frac{16}{\gamma _{1}}B \biggl( \alpha p+1,\frac{1}{\gamma _{1}} \biggr) \biggr) \biggl( 16- \frac{16}{\gamma _{2}}B \biggl( \beta p+1,\frac{1}{\gamma _{2}} \biggr) \biggr) \biggr] ^{{\frac{1}{p}}} \\& \qquad {}\times \biggl[ \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} ( \eta _{1},\vartheta _{1} ) \biggr\vert ^{q}+ \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} ( \eta _{1},\vartheta _{2} ) \biggr\vert ^{q}+ \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} ( \eta _{2}, \vartheta _{1} ) \biggr\vert ^{q}+ \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} ( \eta _{2}, \vartheta _{2} ) \biggr\vert ^{q} \biggr] ^{\frac{1}{q}}, \end{aligned}$$
(2.12)
where A is defined by (2.2), \(B ( \cdot ,\cdot ) \) refers to the Beta function and \(\frac{1}{p}=1-\frac{1}{q}\).
Proof
By using the well-known Hölder’s inequality for double integrals, since \(\vert \frac {\partial ^{2}\xi }{\partial t\partial s} \vert ^{q}\) is convex functions on the co-ordinates on △, we get
$$\begin{aligned}& \int _{0}^{1} \int _{0}^{1} \biggl\vert \frac{1}{\gamma _{1}^{\alpha }}- \biggl( \frac{1-(1-t)^{\gamma _{1}}}{\gamma _{1}} \biggr) ^{\alpha } \biggr\vert \biggl\vert \frac{1}{\gamma _{2}^{\beta }}- \biggl( \frac{1-(1-s)^{\gamma _{2}}}{\gamma _{2}} \biggr) ^{\beta } \biggr\vert \\& \qquad {}\times \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} \biggl( \frac{1+t}{2}\eta _{1}+\frac{1-t}{2}\eta _{2},\frac{1+s}{2} \vartheta _{1}+\frac{1-s}{2}\vartheta _{2} \biggr) \biggr\vert \,ds\,dt \\& \quad \leq \biggl( \int _{0}^{1} \int _{0}^{1} \biggl\vert \frac{1}{\gamma _{1}^{\alpha }}- \biggl( \frac{1-(1-t)^{\gamma _{1}}}{\gamma _{1}} \biggr) ^{\alpha } \biggr\vert ^{p} \biggl\vert \frac{1}{\gamma _{2}^{\beta }}- \biggl( \frac{1-(1-s)^{\gamma _{2}}}{\gamma _{2}} \biggr) ^{\beta } \biggr\vert ^{p}\,ds\,dt \biggr) ^{\frac{1}{p}} \\& \qquad {}\times \biggl( \int _{0}^{1} \int _{0}^{1} \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} \biggl( \frac{1+t}{2}\eta _{1}+ \frac{1-t}{2}\eta _{2},\frac{1+s}{2}\vartheta _{1}+\frac{1-s}{2} \vartheta _{2} \biggr) \biggr\vert ^{q}\,ds\,dt \biggr) ^{\frac{1}{q}} \\& \quad \leq \frac{1}{\gamma _{1}^{\alpha }}\frac{1}{\gamma _{2}^{\beta }} \biggl( \int _{0}^{1} \int _{0}^{1} \bigl( 1- \bigl( 1-(1-t)^{ \gamma _{1}} \bigr) ^{\alpha p} \bigr) \bigl( 1- \bigl( 1-(1-s)^{ \gamma _{2}} \bigr) ^{\beta p} \bigr)\,ds\,dt \biggr) ^{{\frac{1}{p}}} \\& \qquad {}\times \biggl\{ \biggl( \frac{1+t}{2} \biggr) \biggl( \frac{1+s}{2} \biggr) \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} ( \eta _{1},\vartheta _{1} ) \biggr\vert ^{q}+ \biggl( \frac{1+t}{2} \biggr) \biggl( \frac{1-s}{2} \biggr) \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} ( \eta _{1}, \vartheta _{2} ) \biggr\vert ^{q} \\& \qquad {}+ \biggl( \frac{1-t}{2} \biggr) \biggl( \frac{1+s}{2} \biggr) \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} ( \eta _{2},\vartheta _{1} ) \biggr\vert ^{q}+ \biggl( \frac{1-t}{2} \biggr) \biggl( \frac{1-s}{2} \biggr) \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} ( \eta _{2},\vartheta _{2} ) \biggr\vert ^{q}\,ds\,dt \biggr\} ^{{\frac{1}{q}}} \\& \quad \leq \frac{1}{\gamma _{1}^{\alpha }}\frac{1}{\gamma _{2}^{\beta }} \biggl[ \biggl( 1- \frac{1}{\gamma _{1}}B \biggl( \alpha p+1, \frac{1}{\gamma _{1}} \biggr) \biggr) \biggl( 1-\frac{1}{\gamma _{2}}B \biggl( \beta p+1, \frac{1}{\gamma _{2}} \biggr) \biggr) \biggr] ^{{\frac{1}{p}}} \\& \qquad {}\times \biggl( \frac{9}{16} \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} ( \eta _{1}, \vartheta _{1} ) \biggr\vert ^{q}+ \frac{3}{16} \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} ( \eta _{1}, \vartheta _{2} ) \biggr\vert ^{q}+\frac{3}{16} \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} ( \eta _{2},\vartheta _{1} ) \biggr\vert ^{q} \\& \qquad {}+\frac{1}{16} \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} ( \eta _{2}, \vartheta _{2} ) \biggr\vert ^{q} \biggr) ^{{\frac{1}{q}}}. \end{aligned}$$
(2.13)
Here, we take advantage of the fact that
$$ (\varpi -\sigma )^{j}\leq \varpi ^{j}-\sigma ^{j}, $$
for any \(\varpi >\sigma \geq 0\) and \(j\geq 1\).
Similarly, we have
$$\begin{aligned}& \int _{0}^{1} \int _{0}^{1} \biggl\vert \frac{1}{\gamma _{1}^{\alpha }}- \biggl( \frac{1-(1-t)^{\gamma _{1}}}{\gamma _{1}} \biggr) ^{\alpha } \biggr\vert \biggl\vert \frac{1}{\gamma _{2}^{\beta }}- \biggl( \frac{1-(1-s)^{\gamma _{2}}}{\gamma _{2}} \biggr) ^{\beta } \biggr\vert \end{aligned}$$
(2.14)
$$\begin{aligned}& \qquad {}\times \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} \biggl( \frac{1+t}{2}\eta _{1}+\frac{1-t}{2}\eta _{2},\frac{1-s}{2} \vartheta _{1}+\frac{1+s}{2}\vartheta _{2} \biggr) \biggr\vert \,ds\,dt \} \\& \quad \leq \frac{1}{\gamma _{1}^{\alpha }}\frac{1}{\gamma _{2}^{\beta }} \biggl[ \biggl( 1- \frac{1}{\gamma _{1}}B \biggl( \alpha p+1, \frac{1}{\gamma _{1}} \biggr) \biggr) \biggl( 1-\frac{1}{\gamma _{2}}B \biggl( \beta p+1, \frac{1}{\gamma _{2}} \biggr) \biggr) \biggr] ^{{\frac{1}{p}}} \\& \qquad {}\times \biggl( \frac{3}{16} \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} ( \eta _{1}, \vartheta _{1} ) \biggr\vert ^{q}+ \frac{9}{16} \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} ( \eta _{1}, \vartheta _{2} ) \biggr\vert ^{q}+\frac{1}{16} \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} ( \eta _{2},\vartheta _{1} ) \biggr\vert ^{q} \\& \qquad {}+\frac{3}{16} \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} ( \eta _{2}, \vartheta _{2} ) \biggr\vert ^{q} \biggr) ^{{\frac{1}{q}}}, \\& \int _{0}^{1} \int _{0}^{1} \biggl\vert \frac{1}{\gamma _{1}^{\alpha }}- \biggl( \frac{1-(1-t)^{\gamma _{1}}}{\gamma _{1}} \biggr) ^{\alpha } \biggr\vert \biggl\vert \frac{1}{\gamma _{2}^{\beta }}- \biggl( \frac{1-(1-s)^{\gamma _{2}}}{\gamma _{2}} \biggr) ^{\beta } \biggr\vert \\& \qquad {}\times \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} \biggl( \frac{1-t}{2}\eta _{1}+\frac{1+t}{2}\eta _{2},\frac{1+s}{2} \vartheta _{1}+\frac{1-s}{2}\vartheta _{2} \biggr) \biggr\vert \,ds\,dt \} \\& \quad \leq \frac{1}{\gamma _{1}^{\alpha }}\frac{1}{\gamma _{2}^{\beta }} \biggl[ \biggl( 1- \frac{1}{\gamma _{1}}B \biggl( \alpha p+1, \frac{1}{\gamma _{1}} \biggr) \biggr) \biggl( 1-\frac{1}{\gamma _{2}}B \biggl( \beta p+1, \frac{1}{\gamma _{2}} \biggr) \biggr) \biggr] ^{{\frac{1}{p}}} \\& \qquad {}\times \biggl( \frac{3}{16} \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} ( \eta _{1}, \vartheta _{1} ) \biggr\vert ^{q}+ \frac{1}{16} \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} ( \eta _{1}, \vartheta _{2} ) \biggr\vert ^{q}+\frac{9}{16} \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} ( \eta _{2},\vartheta _{1} ) \biggr\vert ^{q} \\& \qquad {}+\frac{3}{16} \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} ( \eta _{2}, \vartheta _{2} ) \biggr\vert ^{q} \biggr) ^{{\frac{1}{q}}}, \end{aligned}$$
(2.15)
and
$$\begin{aligned}& \int _{0}^{1} \int _{0}^{1} \biggl\vert \frac{1}{\gamma _{1}^{\alpha }}- \biggl( \frac{1-(1-t)^{\gamma _{1}}}{\gamma _{1}} \biggr) ^{\alpha } \biggr\vert \biggl\vert \frac{1}{\gamma _{2}^{\beta }}- \biggl( \frac{1-(1-s)^{\gamma _{2}}}{\gamma _{2}} \biggr) ^{\beta } \biggr\vert \\& \qquad {}\times \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} \biggl( \frac{1-t}{2}\eta _{1}+\frac{1+t}{2}\eta _{2},\frac{1-s}{2} \vartheta _{1}+\frac{1+s}{2}\vartheta _{2} \biggr) \biggr\vert \,ds\,dt \\& \quad \leq \frac{1}{\gamma _{1}^{\alpha }}\frac{1}{\gamma _{2}^{\beta }} \biggl[ \biggl( 1- \frac{1}{\gamma _{1}}B \biggl( \alpha p+1, \frac{1}{\gamma _{1}} \biggr) \biggr) \biggl( 1-\frac{1}{\gamma _{2}}B \biggl( \beta p+1, \frac{1}{\gamma _{2}} \biggr) \biggr) \biggr] ^{{\frac{1}{p}}} \\& \qquad {}\times \biggl( \frac{1}{16} \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} ( \eta _{1}, \vartheta _{1} ) \biggr\vert ^{q}+ \frac{3}{16} \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} ( \eta _{1}, \vartheta _{2} ) \biggr\vert ^{q}+\frac{3}{16} \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} ( \eta _{2},\vartheta _{1} ) \biggr\vert ^{q} \\& \qquad {}+\frac{9}{16} \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} ( \eta _{2}, \vartheta _{2} ) \biggr\vert ^{q} \biggr) ^{{\frac{1}{q}}}. \end{aligned}$$
(2.16)
If we substitute the inequalities (2.13)–(2.16) in (2.9), we obtain the desired inequality (2.12). □
Remark 5
If we take \(\gamma _{1}=1\) and \(\gamma _{2}=1\) in Theorem 4, then the following inequality for Riemann–Liouville fractional integrals is achieved
$$\begin{aligned}& \biggl\vert \xi \biggl( \frac{\eta _{1}+\eta _{2}}{2}, \frac{\vartheta _{1}+\vartheta _{2}}{2} \biggr) + \frac{2^{\alpha -1}2^{\beta -1}\Gamma (\alpha +1)\Gamma (\beta +1)}{ ( \eta _{2}-\eta _{1} ) ^{\alpha } ( \vartheta _{2}-\vartheta _{1} ) ^{\beta }} \\& \qquad {}\times \biggl[ J_{\eta _{1}^{+},\vartheta _{1}^{+}}^{\alpha ,\beta }\xi \biggl( \frac{\eta _{1}+\eta _{2}}{2}, \frac{\vartheta _{1}+\vartheta _{2}}{2} \biggr) \\& \qquad {}+ J_{\eta _{1}^{+},\vartheta _{2}^{-}}^{\alpha , \beta }\xi \biggl( \frac{\eta _{1}+\eta _{2}}{2}, \frac{\vartheta _{1}+\vartheta _{2}}{2} \biggr) +J_{\eta _{2}^{-},\vartheta _{1}^{+}}^{\alpha ,\beta } \xi \biggl( \frac{\eta _{1}+\eta _{2}}{2}, \frac{\vartheta _{1}+\vartheta _{2}}{2} \biggr) \\& \qquad {}+J_{\eta _{2}^{-}, \vartheta _{2}^{-}}^{\alpha ,\beta } \xi \biggl( \frac{\eta _{1}+\eta _{2}}{2},\frac{\vartheta _{1}+\vartheta _{2}}{2} \biggr) \biggr] -D \biggr\vert \\& \quad \leq \frac{ ( \eta _{2}-\eta _{1} ) ( \vartheta _{2}-\vartheta _{1} ) }{16} \biggl[ \biggl( \frac{16\alpha p}{\alpha p+1} \biggr) \biggl( \frac{16\beta p}{\beta p+1} \biggr) \biggr] ^{{ \frac{1}{p}}} \\& \qquad {}\times \biggl[ \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} ( \eta _{1},\vartheta _{1} ) \biggr\vert ^{q}+ \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} ( \eta _{1},\vartheta _{2} ) \biggr\vert ^{q}+ \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} ( \eta _{2}, \vartheta _{1} ) \biggr\vert ^{q}+ \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} ( \eta _{2}, \vartheta _{2} ) \biggr\vert ^{q} \biggr] ^{\frac{1}{q}}. \end{aligned}$$
(2.17)
Theorem 5
Assume that the assumptions of Lemma 1hold. If \(\vert \frac {\partial ^{2}\xi }{\partial t\partial s} \vert ^{q} \), \(q\geq 1\), is a co-ordinated convex function on Δ, then we have the following inequality:
$$\begin{aligned}& \biggl\vert \xi \biggl( \frac{\eta _{1}+\eta _{2}}{2}, \frac{\vartheta _{1}+\vartheta _{2}}{2} \biggr) + \frac{2^{\gamma _{1}\alpha -1}2^{\gamma _{2}\beta -1}\Gamma (\alpha +1)\Gamma (\beta +1)\gamma _{1}^{\alpha } \gamma _{2}^{\beta }}{ ( \eta _{2}-\eta _{1} ) ^{\gamma _{1}\alpha } ( \vartheta _{2}-\vartheta _{1} ) ^{\gamma _{2}\beta }} \\& \qquad {}\times \biggl[ ^{ \gamma _{1}\gamma _{2}}I_{\eta _{1}^{+},\vartheta _{1}^{+}}^{\alpha , \beta }\xi \biggl( \frac{\eta _{1}+\eta _{2}}{2}, \frac{\vartheta _{1}+\vartheta _{2}}{2} \biggr) \\& \qquad {}+ ^{\gamma _{1}\gamma _{2}}I_{\eta _{1}^{+}, \vartheta _{2}^{-}}^{\alpha ,\beta }\xi \biggl( \frac{\eta _{1}+\eta _{2}}{2}, \frac{\vartheta _{1}+\vartheta _{2}}{2} \biggr) +{}^{\gamma _{1} \gamma _{2}}I_{\eta _{2}^{-},\vartheta _{1}^{+}}^{\alpha ,\beta } \xi \biggl( \frac{\eta _{1}+\eta _{2}}{2},\frac{\vartheta _{1}+\vartheta _{2}}{2} \biggr) \\& \qquad {} +{}^{\gamma _{1}\gamma _{2}}I_{\eta _{2}^{-},\vartheta _{2}^{-}}^{ \alpha ,\beta }\xi \biggl( \frac{\eta _{1}+\eta _{2}}{2}, \frac{\vartheta _{1}+\vartheta _{2}}{2} \biggr) \biggr] -A \biggr\vert \\& \quad \leq \frac{ ( \eta _{2}-\eta _{1} ) ( \vartheta _{2}-\vartheta _{1} ) }{\gamma _{1}\gamma _{2}} \biggl( \frac{1}{4} \biggr) ^{2+\frac{1}{q}} \biggl[ \biggl( 1-\frac{1}{\gamma _{1}}B \biggl( \alpha +1, \frac{1}{\gamma _{1}} \biggr) \biggr) \biggl( 1- \frac{1}{\gamma _{2}}B \biggl( \beta +1,\frac{1}{\gamma _{2}} \biggr) \biggr) \biggr] ^{1- \frac{1}{q}} \\& \qquad {}\times \bigg\{ \biggl( \biggl[ \frac{3\gamma _{1}}{2}-2B \biggl( \alpha +1,\frac{1}{\gamma _{1}} \biggr) +B \biggl( \alpha +1, \frac{2}{\gamma _{1}} \biggr) \biggr] \\& \qquad {}\times \biggl[ \frac{3\gamma _{2}}{2}-2B \biggl( \beta +1,\frac{1}{\gamma _{2}} \biggr) +B \biggl( \beta +1,\frac{2}{\gamma _{2}} \biggr) \biggr] \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} ( \eta _{1},\vartheta _{1} ) \biggr\vert ^{q} \\& \qquad {}+ \biggl[ \frac{3\gamma _{1}}{2}-2B \biggl( \alpha +1,\frac{1}{\gamma _{1}} \biggr) +B \biggl( \alpha +1,\frac{2}{\gamma _{1}} \biggr) \biggr] \biggl[ \frac{\gamma _{2}}{2}-B \biggl( \beta +1, \frac{2}{\gamma _{2}} \biggr) \biggr] \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} ( \eta _{1}, \vartheta _{2} ) \biggr\vert ^{q} \\& \qquad {}+ \biggl[ \frac{\gamma _{1}}{2}-B \biggl( \alpha +1, \frac{2}{\gamma _{1}} \biggr) \biggr] \biggl[ \frac{3\gamma _{2}}{2}-2B \biggl( \beta +1,\frac{1}{\gamma _{2}} \biggr) +B \biggl( \beta +1, \frac{2}{\gamma _{2}} \biggr) \biggr] \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} ( \eta _{2}, \vartheta _{1} ) \biggr\vert ^{q} \\& \qquad {}+ \biggl[ \frac{\gamma _{1}}{2}-B \biggl( \alpha +1, \frac{2}{\gamma _{1}} \biggr) \biggr] \biggl[ \frac{\gamma _{2}}{2}-B \biggl( \beta +1,\frac{2}{\gamma _{2}} \biggr) \biggr] \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} ( \eta _{2},\vartheta _{2} ) \biggr\vert ^{q} \biggr) ^{\frac{1}{q}} \\& \qquad {}+ \biggl( \biggl[ \frac{3\gamma _{1}}{2}-2B \biggl( \alpha +1, \frac{1}{\gamma _{1}} \biggr) +B \biggl( \alpha +1,\frac{2}{\gamma _{1}} \biggr) \biggr] \biggl[ \frac{\gamma _{2}}{2}-B \biggl( \beta +1, \frac{2}{\gamma _{2}} \biggr) \biggr] \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} ( \eta _{1}, \vartheta _{1} ) \biggr\vert ^{q} \\& \qquad {}+ \biggl[ \frac{3\gamma _{1}}{2}-2B \biggl( \alpha +1,\frac{1}{\gamma _{1}} \biggr) +B \biggl( \alpha +1,\frac{2}{\gamma _{1}} \biggr) \biggr] \\& \qquad {}\times \biggl[ \frac{3\gamma _{2}}{2}-2B \biggl( \beta +1, \frac{1}{\gamma _{2}} \biggr) +B \biggl( \beta +1, \frac{2}{\gamma _{2}} \biggr) \biggr] \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} ( \eta _{1},\vartheta _{2} ) \biggr\vert ^{q} \\& \qquad {}+ \biggl[ \frac{\gamma _{1}}{2}-B \biggl( \alpha +1, \frac{2}{\gamma _{1}} \biggr) \biggr] \biggl[ \frac{\gamma _{2}}{2}-B \biggl( \beta +1,\frac{2}{\gamma _{2}} \biggr) \biggr] \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} ( \eta _{2},\vartheta _{1} ) \biggr\vert ^{q} \\& \qquad {}+ \biggl[ \frac{\gamma _{1}}{2}-B \biggl( \alpha +1, \frac{2}{\gamma _{1}} \biggr) \biggr] \\& \qquad {}\times \biggl[ \frac{3\gamma _{2}}{2}-2B \biggl( \beta +1,\frac{1}{\gamma _{2}} \biggr) +B \biggl( \beta +1, \frac{2}{\gamma _{2}} \biggr) \biggr] \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} ( \eta _{2}, \vartheta _{2} ) \biggr\vert ^{q} \biggr) ^{{\frac{1}{q}}} \\& \qquad {}+ \biggl( \biggl[ \frac{\gamma _{1}}{2}-B \biggl( \alpha +1, \frac{2}{\gamma _{1}} \biggr) \biggr] \biggl[ \frac{3\gamma _{2}}{2}-2B \biggl( \beta +1,\frac{1}{\gamma _{2}} \biggr) +B \biggl( \beta +1,\frac{2}{\gamma _{2}} \biggr) \biggr] \\& \qquad {}\times \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} ( \eta _{1}, \vartheta _{1} ) \biggr\vert ^{q} \\& \qquad {}+ \biggl[ \frac{\gamma _{1}}{2}-B \biggl( \alpha +1, \frac{2}{\gamma _{1}} \biggr) \biggr] \biggl[ \frac{\gamma _{2}}{2}-B \biggl( \beta +1,\frac{2}{\gamma _{2}} \biggr) \biggr] \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} ( \eta _{1},\vartheta _{2} ) \biggr\vert ^{q} \\& \qquad {}+ \biggl[ \frac{3\gamma _{1}}{2}-2B \biggl( \alpha +1,\frac{1}{\gamma _{1}} \biggr) +B \biggl( \alpha +1,\frac{2}{\gamma _{1}} \biggr) \biggr] \\& \qquad {}\times \biggl[ \frac{3\gamma _{2}}{2}-2B \biggl( \beta +1, \frac{1}{\gamma _{2}} \biggr) +B \biggl( \beta +1, \frac{2}{\gamma _{2}} \biggr) \biggr] \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} ( \eta _{2},\vartheta _{1} ) \biggr\vert ^{q} \\& \qquad {}+ \biggl[ \frac{3\gamma _{1}}{2}-2B \biggl( \alpha +1,\frac{1}{\gamma _{1}} \biggr) +B \biggl( \alpha +1,\frac{2}{\gamma _{1}} \biggr) \biggr] \\& \qquad {}\times \biggl[ \frac{\gamma _{2}}{2}-B \biggl( \beta +1, \frac{2}{\gamma _{2}} \biggr) \biggr] \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} ( \eta _{2}, \vartheta _{2} ) \biggr\vert ^{q} \biggr) ^{{\frac{1}{q}}} \\& \qquad {}+ \biggl( \biggl[ \frac{\gamma _{1}}{2}-B \biggl( \alpha +1, \frac{2}{\gamma _{1}} \biggr) \biggr] \biggl[ \frac{\gamma _{2}}{2}-B \biggl( \beta +1,\frac{2}{\gamma _{2}} \biggr) \biggr] \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} ( \eta _{1}, \vartheta _{1} ) \biggr\vert ^{q} \\& \qquad {}+ \biggl[ \frac{\gamma _{1}}{2}-B \biggl( \alpha +1, \frac{2}{\gamma _{1}} \biggr) \biggr] \biggl[ \frac{3\gamma _{2}}{2}-2B \biggl( \beta +1,\frac{1}{\gamma _{2}} \biggr) +B \biggl( \beta +1, \frac{2}{\gamma _{2}} \biggr) \biggr] \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} ( \eta _{1}, \vartheta _{2} ) \biggr\vert ^{q} \\& \qquad {}+ \biggl[ \frac{3\gamma _{1}}{2}-2B \biggl( \alpha +1,\frac{1}{\gamma _{1}} \biggr) +B \biggl( \alpha +1,\frac{2}{\gamma _{1}} \biggr) \biggr] \biggl[ \frac{\gamma _{2}}{2}-B \biggl( \beta +1, \frac{2}{\gamma _{2}} \biggr) \biggr] \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} ( \eta _{2}, \vartheta _{1} ) \biggr\vert ^{q} \\& \qquad {}+ \biggl[ \frac{3\gamma _{1}}{2}-2B \biggl( \alpha +1,\frac{1}{\gamma _{1}} \biggr) +B \biggl( \alpha +1,\frac{2}{\gamma _{1}} \biggr) \biggr] \\& \qquad {}\times \biggl[ \frac{3\gamma _{2}}{2}-2B \biggl( \beta +1, \frac{1}{\gamma _{2}} \biggr) +B \biggl( \beta +1, \frac{2}{\gamma _{2}} \biggr) \biggr] \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} ( \eta _{2},\vartheta _{2} ) \biggr\vert ^{q} \biggr) ^{{{\frac{1}{q}}}}. \end{aligned}$$
(2.18)
Here, A is defined as in (2.2).
Proof
By using power-mean inequality, we get
$$\begin{aligned} I_{9} =& \int _{0}^{1} \int _{0}^{1} \biggl\vert \frac{1}{\gamma _{1}^{\alpha }}- \biggl( \frac{1-(1-t)^{\gamma _{1}}}{\gamma _{1}} \biggr) ^{\alpha } \biggr\vert \biggl\vert \frac{1}{\gamma _{2}^{\beta }}- \biggl( \frac{1-(1-s)^{\gamma _{2}}}{\gamma _{2}} \biggr) ^{\beta } \biggr\vert \\ &{}\times \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} \biggl( \frac{1+t}{2}\eta _{1}+\frac{1-t}{2}\eta _{2},\frac{1+s}{2} \vartheta _{1}+\frac{1-s}{2}\vartheta _{2} \biggr) \biggr\vert \,ds\,dt \\ \leq & \biggl( \int _{0}^{1} \int _{0}^{1} \biggl\vert \frac{1}{\gamma _{1}^{\alpha }}- \biggl( \frac{1-(1-t)^{\gamma _{1}}}{\gamma _{1}} \biggr) ^{\alpha } \biggr\vert \biggl\vert \frac{1}{\gamma _{2}^{\beta }}- \biggl( \frac{1-(1-s)^{\gamma _{2}}}{\gamma _{2}} \biggr) ^{\beta } \biggr\vert \,ds\,dt \biggr) ^{1-\frac{1}{q}} \\ &{}\times \biggl( \int _{0}^{1} \int _{0}^{1} \biggl\vert \frac{1}{\gamma _{1}^{\alpha }}- \biggl( \frac{1-(1-t)^{\gamma _{1}}}{\gamma _{1}} \biggr) ^{\alpha } \biggr\vert \biggl\vert \frac{1}{\gamma _{2}^{\beta }}- \biggl( \frac{1-(1-s)^{\gamma _{2}}}{\gamma _{2}} \biggr) ^{\beta } \biggr\vert \\ & {} \times \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} \biggl( \frac{1+t}{2}\eta _{1}+\frac{1-t}{2}\eta _{2}, \frac{1+s}{2} \vartheta _{1}+\frac{1-s}{2}\vartheta _{2} \biggr) \biggr\vert ^{q}\,ds\,dt \biggr) ^{\frac{1}{q}}. \end{aligned}$$
Taking into account co-ordinated convexity of \(\vert \frac {\partial ^{2}\xi }{\partial t\partial s} \vert ^{q}\), we acquire
$$\begin{aligned}& \int _{0}^{1} \int _{0}^{1} \biggl\vert \frac{1}{\gamma _{1}^{\alpha }}- \biggl( \frac{1-(1-t)^{\gamma _{1}}}{\gamma _{1}} \biggr) ^{\alpha } \biggr\vert \biggl\vert \frac{1}{\gamma _{2}^{\beta }}- \biggl( \frac{1-(1-s)^{\gamma _{2}}}{\gamma _{2}} \biggr) ^{\beta } \biggr\vert \\& \qquad {}\times \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} \biggl( \frac{1+t}{2}\eta _{1}+\frac{1-t}{2}\eta _{2},\frac{1+s}{2} \vartheta _{1}+\frac{1-s}{2}\vartheta _{2} \biggr) \biggr\vert \,ds\,dt \\& \quad \leq \biggl( \int _{0}^{1} \int _{0}^{1} \biggl( \frac{1}{\gamma _{1}^{\alpha }}- \biggl( \frac{1-(1-t)^{\gamma _{1}}}{\gamma _{1}} \biggr) ^{\alpha } \biggr) \biggl( \frac{1}{\gamma _{2}^{\beta }}- \biggl( \frac{1-(1-s)^{\gamma _{2}}}{\gamma _{2}} \biggr) ^{\beta } \biggr)\,ds\,dt \biggr) ^{1-\frac{1}{q}} \\& \qquad {}\times \bigg( \int _{0}^{1} \int _{0}^{1} \biggl( \frac{1}{\gamma _{1}^{\alpha }}- \biggl( \frac{1-(1-t)^{\gamma _{1}}}{\gamma _{1}} \biggr) ^{\alpha } \biggr) \biggl( \frac{1}{\gamma _{2}^{\beta }}- \biggl( \frac{1-(1-s)^{\gamma _{2}}}{\gamma _{2}} \biggr) ^{\beta } \biggr) \\& \qquad {}\times \biggl\{ \biggl( \frac{1+t}{2} \biggr) \biggl( \frac{1+s}{2} \biggr) \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} ( \eta _{1},\vartheta _{1} ) \biggr\vert ^{q}+ \biggl( \frac{1+t}{2} \biggr) \biggl( \frac{1-s}{2} \biggr) \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} ( \eta _{1}, \vartheta _{2} ) \biggr\vert ^{q} \\& \qquad {}+ \biggl( \frac{1-t}{2} \biggr) \biggl( \frac{1+s}{2} \biggr) \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} ( \eta _{2},\vartheta _{1} ) \biggr\vert ^{q}+ \biggl( \frac{1-t}{2} \biggr) \biggl( \frac{1-s}{2} \biggr) \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} ( \eta _{2}, \vartheta _{2} ) \biggr\vert ^{q}\,ds\,dt \biggr\} ^{{\frac{1}{q}}} \\& \quad = \biggl[ \frac{1}{\gamma _{1}^{\alpha }} \frac{1}{\gamma _{2}^{\beta }} \biggl( 1- \frac{1}{\gamma _{1}}B \biggl( \alpha +1, \frac{1}{\gamma _{1}} \biggr) \biggr) \biggl( 1- \frac{1}{\gamma _{2}}B \biggl( \beta +1,\frac{1}{\gamma _{2}} \biggr) \biggr) \biggr] ^{1-\frac{1}{q}} \biggl\{ \frac{1}{4} \frac{1}{\gamma _{1}^{\alpha +1}} \frac{1}{\gamma _{2}^{\beta +1}} \\& \qquad {}\times \biggl( \biggl[ \frac{3\gamma _{1}}{2}-2B \biggl( \alpha +1, \frac{1}{\gamma _{1}} \biggr) +B \biggl( \alpha +1,\frac{2}{\gamma _{1}} \biggr) \biggr] \\& \qquad {} \times \biggl[ \frac{3\gamma _{2}}{2}-2B \biggl( \beta +1, \frac{1}{\gamma _{2}} \biggr) +B \biggl( \beta +1, \frac{2}{\gamma _{2}} \biggr) \biggr] \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} ( \eta _{1},\vartheta _{1} ) \biggr\vert ^{q} \\& \qquad {}+ \biggl[ \frac{3\gamma _{1}}{2}-2B \biggl( \alpha +1, \frac{1}{\gamma _{1}} \biggr) +B \biggl( \alpha +1, \frac{2}{\gamma _{1}} \biggr) \biggr] \biggl[ \frac{\gamma _{2}}{2}-B \biggl( \beta +1,\frac{2}{\gamma _{2}} \biggr) \biggr] \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} ( \eta _{1}, \vartheta _{2} ) \biggr\vert ^{q} \\& \qquad {}+ \biggl[ \frac{\gamma _{1}}{2}-B \biggl( \alpha +1, \frac{2}{\gamma _{1}} \biggr) \biggr] \biggl[ \frac{3\gamma _{2}}{2}-2B \biggl( \beta +1, \frac{1}{\gamma _{2}} \biggr) +B \biggl( \beta +1,\frac{2}{\gamma _{2}} \biggr) \biggr] \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} ( \eta _{2}, \vartheta _{1} ) \biggr\vert ^{q} \\& \qquad {}+ \biggl[ \frac{\gamma _{1}}{2}-B \biggl( \alpha +1, \frac{2}{\gamma _{1}} \biggr) \biggr] \biggl[ \frac{\gamma _{2}}{2}-B \biggl( \beta +1,\frac{2}{\gamma _{2}} \biggr) \biggr] \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} ( \eta _{2},\vartheta _{2} ) \biggr\vert ^{q} \biggr) \biggr\} ^{\frac{1}{q}}. \end{aligned}$$
(2.19)
Similarly, we have
$$\begin{aligned}& \int _{0}^{1} \int _{0}^{1} \biggl( \frac{1}{\gamma _{1}^{\alpha }}- \biggl( \frac{1-(1-t)^{\gamma _{1}}}{\gamma _{1}} \biggr) ^{\alpha } \biggr) \biggl( \frac{1}{\gamma _{2}^{\beta }}- \biggl( \frac{1-(1-s)^{\gamma _{2}}}{\gamma _{2}} \biggr) ^{\beta } \biggr) \end{aligned}$$
(2.20)
$$\begin{aligned}& \qquad {}\times \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} \biggl( \frac{1+t}{2}\eta _{1}+\frac{1-t}{2}\eta _{2},\frac{1-s}{2} \vartheta _{1}+\frac{1+s}{2}\vartheta _{2} \biggr) \biggr\vert \,ds\,dt \\& \quad \leq \biggl[ \frac{1}{\gamma _{1}^{\alpha }} \frac{1}{\gamma _{2}^{\beta }} \biggl( 1- \frac{1}{\gamma _{1}}B \biggl( \alpha +1, \frac{1}{\gamma _{1}} \biggr) \biggr) \biggl( 1- \frac{1}{\gamma _{2}}B \biggl( \beta +1,\frac{1}{\gamma _{2}} \biggr) \biggr) \biggr] ^{1-\frac{1}{q}} \biggl\{ \frac{1}{4} \frac{1}{\gamma _{1}^{\alpha +1}} \frac{1}{\gamma _{2}^{\beta +1}} \\& \qquad {}\times \biggl( \biggl[ \frac{3\gamma _{1}}{2}-2B \biggl( \alpha +1, \frac{1}{\gamma _{1}} \biggr) +B \biggl( \alpha +1,\frac{2}{\gamma _{1}} \biggr) \biggr] \\& \qquad {}\times \biggl[ \frac{\gamma _{2}}{2}-B \biggl( \beta +1, \frac{2}{\gamma _{2}} \biggr) \biggr] \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} ( \eta _{1}, \vartheta _{1} ) \biggr\vert ^{q} \\& \qquad {}+ \biggl[ \frac{3\gamma _{1}}{2}-2B \biggl( \alpha +1, \frac{1}{\gamma _{1}} \biggr) +B \biggl( \alpha +1, \frac{2}{\gamma _{1}} \biggr) \biggr] \\& \qquad {}\times \biggl[ \frac{3\gamma _{2}}{2}-2B \biggl( \beta +1,\frac{1}{\gamma _{2}} \biggr) +B \biggl( \beta +1, \frac{2}{\gamma _{2}} \biggr) \biggr] \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} ( \eta _{1},\vartheta _{2} ) \biggr\vert ^{q} \\& \qquad {}+ \biggl[ \frac{\gamma _{1}}{2}-B \biggl( \alpha +1, \frac{2}{\gamma _{1}} \biggr) \biggr] \biggl[ \frac{\gamma _{2}}{2}-B \biggl( \beta +1, \frac{2}{\gamma _{2}} \biggr) \biggr] \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} ( \eta _{2}, \vartheta _{1} ) \biggr\vert ^{q} \\& \qquad {}+ \biggl[ \frac{\gamma _{1}}{2}-B \biggl( \alpha +1, \frac{2}{\gamma _{1}} \biggr) \biggr] \biggl[ \frac{3\gamma _{2}}{2}-2B \biggl( \beta +1,\frac{1}{\gamma _{2}} \biggr) +B \biggl( \beta +1, \frac{2}{\gamma _{2}} \biggr) \biggr] \\& \qquad {}\times \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} ( \eta _{2}, \vartheta _{2} ) \biggr\vert ^{q} \biggr) \biggr\} ^{ \frac{1}{q}}, \\& \int _{0}^{1} \int _{0}^{1} \biggl( \frac{1}{\gamma _{1}^{\alpha }}- \biggl( \frac{1-(1-t)^{\gamma _{1}}}{\gamma _{1}} \biggr) ^{\alpha } \biggr) \biggl( \frac{1}{\gamma _{2}^{\beta }}- \biggl( \frac{1-(1-s)^{\gamma _{2}}}{\gamma _{2}} \biggr) ^{\beta } \biggr) \\& \qquad {}\times \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} \biggl( \frac{1-t}{2}\eta _{1}+\frac{1+t}{2}\eta _{2},\frac{1+s}{2} \vartheta _{1}+\frac{1-s}{2}\vartheta _{2} \biggr) \biggr\vert \,ds\,dt \\& \quad \leq \biggl[ \frac{1}{\gamma _{1}^{\alpha }} \frac{1}{\gamma _{2}^{\beta }} \biggl( 1- \frac{1}{\gamma _{1}}B \biggl( \alpha +1, \frac{1}{\gamma _{1}} \biggr) \biggr) \biggl( 1- \frac{1}{\gamma _{2}}B \biggl( \beta +1,\frac{1}{\gamma _{2}} \biggr) \biggr) \biggr] ^{1-\frac{1}{q}} \biggl\{ \frac{1}{4} \frac{1}{\gamma _{1}^{\alpha +1}} \frac{1}{\gamma _{2}^{\beta +1}} \\& \qquad {}\times \biggl( \biggl[ \frac{\gamma _{1}}{2}-B \biggl( \alpha +1, \frac{2}{\gamma _{1}} \biggr) \biggr] \biggl[ \frac{3\gamma _{2}}{2}-2B \biggl( \beta +1,\frac{1}{\gamma _{2}} \biggr) +B \biggl( \beta +1, \frac{2}{\gamma _{2}} \biggr) \biggr] \\& \qquad {}\times \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} ( \eta _{1}, \vartheta _{1} ) \biggr\vert ^{q} \\& \qquad {}+ \biggl[ \frac{\gamma _{1}}{2}-B \biggl( \alpha +1, \frac{2}{\gamma _{1}} \biggr) \biggr] \biggl[ \frac{\gamma _{2}}{2}-B \biggl( \beta +1, \frac{2}{\gamma _{2}} \biggr) \biggr] \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} ( \eta _{1}, \vartheta _{2} ) \biggr\vert ^{q} \\& \qquad {}+ \biggl[ \frac{3\gamma _{1}}{2}-2B \biggl( \alpha +1, \frac{1}{\gamma _{1}} \biggr) +B \biggl( \alpha +1, \frac{2}{\gamma _{1}} \biggr) \biggr] \\& \qquad {}\times \biggl[ \frac{3\gamma _{2}}{2}-2B \biggl( \beta +1,\frac{1}{\gamma _{2}} \biggr) +B \biggl( \beta +1, \frac{2}{\gamma _{2}} \biggr) \biggr] \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} ( \eta _{2},\vartheta _{1} ) \biggr\vert ^{q} \\& \qquad {}+ \biggl[ \frac{3\gamma _{1}}{2}-2B \biggl( \alpha +1, \frac{1}{\gamma _{1}} \biggr) +B \biggl( \alpha +1,\frac{2}{\gamma _{1}} \biggr) \biggr] \\& \qquad {}\times \biggl[ \frac{\gamma _{2}}{2}-B \biggl( \beta +1, \frac{2}{\gamma _{2}} \biggr) \biggr] \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} ( \eta _{2}, \vartheta _{2} ) \biggr\vert ^{q} \biggr) \biggr\} ^{ \frac{1}{q}} \end{aligned}$$
(2.21)
and
$$\begin{aligned}& \int _{0}^{1} \int _{0}^{1} \biggl( \frac{1}{\gamma _{1}^{\alpha }}- \biggl( \frac{1-(1-t)^{\gamma _{1}}}{\gamma _{1}} \biggr) ^{\alpha } \biggr) \biggl( \frac{1}{\gamma _{2}^{\beta }}- \biggl( \frac{1-(1-s)^{\gamma _{2}}}{\gamma _{2}} \biggr) ^{\beta } \biggr) \\& \qquad {}\times \biggl\vert \frac {\partial ^{2}f}{\partial t\partial s} \biggl( \frac{1-t}{2}a+ \frac{1+t}{2}b,\frac{1-s}{2}c+\frac{1+s}{2}\,d\biggr) \biggr\vert \,ds\,dt \\& \quad \leq \biggl[ \frac{1}{\gamma _{1}^{\alpha }} \frac{1}{\gamma _{2}^{\beta }} \biggl( 1- \frac{1}{\gamma _{1}}B \biggl( \alpha +1, \frac{1}{\gamma _{1}} \biggr) \biggr) \biggl( 1- \frac{1}{\gamma _{2}}B \biggl( \beta +1,\frac{1}{\gamma _{2}} \biggr) \biggr) \biggr] ^{1-\frac{1}{q}} \biggl\{ \frac{1}{4} \frac{1}{\gamma _{1}^{\alpha +1}} \frac{1}{\gamma _{2}^{\beta +1}} \\& \qquad {}\times \biggl( \biggl[ \frac{\gamma _{1}}{2}-B \biggl( \alpha +1, \frac{2}{\gamma _{1}} \biggr) \biggr] \biggl[ \frac{\gamma _{2}}{2}-B \biggl( \beta +1,\frac{2}{\gamma _{2}} \biggr) \biggr] \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} ( \eta _{1},\vartheta _{1} ) \biggr\vert ^{q} \\& \qquad {}+ \biggl[ \frac{\gamma _{1}}{2}-B \biggl( \alpha +1, \frac{2}{\gamma _{1}} \biggr) \biggr] \biggl[ \frac{3\gamma _{2}}{2}-2B \biggl( \beta +1, \frac{1}{\gamma _{2}} \biggr) +B \biggl( \beta +1,\frac{2}{\gamma _{2}} \biggr) \biggr] \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} ( \eta _{1}, \vartheta _{2} ) \biggr\vert ^{q} \\& \qquad {}+ \biggl[ \frac{3\gamma _{1}}{2}-2B \biggl( \alpha +1, \frac{1}{\gamma _{1}} \biggr) +B \biggl( \alpha +1, \frac{2}{\gamma _{1}} \biggr) \biggr] \biggl[ \frac{\gamma _{2}}{2}-B \biggl( \beta +1,\frac{2}{\gamma _{2}} \biggr) \biggr] \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} ( \eta _{2}, \vartheta _{1} ) \biggr\vert ^{q} \\& \qquad {}+ \biggl[ \frac{3\gamma _{1}}{2}-2B \biggl( \alpha +1, \frac{1}{\gamma _{1}} \biggr) +B \biggl( \alpha +1,\frac{2}{\gamma _{1}} \biggr) \biggr] \\& \qquad {}\times \biggl[ \frac{3\gamma _{2}}{2}-2B \biggl( \beta +1, \frac{1}{\gamma _{2}} \biggr) +B \biggl( \beta +1, \frac{2}{\gamma _{2}} \biggr) \biggr] \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} ( \eta _{2},\vartheta _{2} ) \biggr\vert ^{q} \biggr) \biggr\} ^{\frac{1}{q}}. \end{aligned}$$
(2.22)
By considering (2.19)–(2.22) in (2.9), we obtain the required inequality (2.18). □
Remark 6
If we take \(\gamma _{1}=1\) and \(\gamma _{2}=1\) in Theorem 5, then the following inequality for Riemann–Liouville fractional integrals is achieved
$$\begin{aligned}& \biggl\vert \xi \biggl( \frac{\eta _{1}+\eta _{2}}{2}, \frac{\vartheta _{1}+\vartheta _{2}}{2} \biggr) + \frac{2^{\alpha -1}2^{\beta -1}\Gamma (\alpha +1)\Gamma (\beta +1)}{ ( \eta _{2}-\eta _{1} ) ^{\alpha } ( \vartheta _{2}-\vartheta _{1} ) ^{\beta }} \\& \qquad {}\times \biggl[ J_{\eta _{1}^{+},\vartheta _{1}^{+}}^{\alpha ,\beta }\xi \biggl( \frac{\eta _{1}+\eta _{2}}{2}, \frac{\vartheta _{1}+\vartheta _{2}}{2} \biggr) \\& \qquad {}+ J_{\eta _{1}^{+},\vartheta _{2}^{-}}^{\alpha , \beta }\xi \biggl( \frac{\eta _{1}+\eta _{2}}{2}, \frac{\vartheta _{1}+\vartheta _{2}}{2} \biggr) +J_{\eta _{2}^{-},\vartheta _{1}^{+}}^{\alpha ,\beta } \xi \biggl( \frac{\eta _{1}+\eta _{2}}{2}, \frac{\vartheta _{1}+\vartheta _{2}}{2} \biggr) \\& \qquad {} +J_{\eta _{2}^{-}, \vartheta _{2}^{-}}^{\alpha ,\beta } \xi \biggl( \frac{\eta _{1}+\eta _{2}}{2},\frac{\vartheta _{1}+\vartheta _{2}}{2} \biggr) \biggr] -D \biggr\vert \\& \quad \leq ( \eta _{2}-\eta _{1} ) ( \vartheta _{2}- \vartheta _{1} ) \biggl( \frac{1}{4} \biggr) ^{2+\frac{1}{q}} \biggl[ \biggl( \frac{\alpha }{\alpha +1} \biggr) \biggl( \frac{\beta }{\beta +1} \biggr) \biggr] ^{1-\frac{1}{q}} \\& \qquad {}\times \bigg\{ \biggl( \biggl[ \frac{3}{2}- \frac{2\alpha +3}{ ( \alpha +1 ) ( \alpha +2 ) } \biggr] \biggl[ \frac{3}{2}- \frac{2\beta +3}{ ( \beta +1 ) ( \beta +2 ) } \biggr] \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} ( \eta _{1},\vartheta _{1} ) \biggr\vert ^{q} \\& \qquad {}+ \biggl[ \frac{3}{2}- \frac{2\alpha +3}{ ( \alpha +1 ) ( \alpha +2 ) } \biggr] \biggl[ \frac{1}{2}- \frac{1}{ ( \beta +1 ) ( \beta +2 ) } \biggr] \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} ( \eta _{1},\vartheta _{2} ) \biggr\vert ^{q} \\& \qquad {}+ \biggl[ \frac{1}{2}- \frac{1}{ ( \alpha +1 ) ( \alpha +2 ) } \biggr] \biggl[ \frac{3}{2}- \frac{2\beta +3}{ ( \beta +1 ) ( \beta +2 ) } \biggr] \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} ( \eta _{2},\vartheta _{1} ) \biggr\vert ^{q} \\& \qquad {}+ \biggl[ \frac{1}{2}- \frac{1}{ ( \alpha +1 ) ( \alpha +2 ) } \biggr] \biggl[ \frac{1}{2}- \frac{1}{ ( \beta +1 ) ( \beta +2 ) } \biggr] \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} ( \eta _{2},\vartheta _{2} ) \biggr\vert ^{q} \biggr) ^{\frac{1}{q}} \\& \qquad {}+ \biggl( \biggl[ \frac{3}{2}- \frac{2\alpha +3}{ ( \alpha +1 ) ( \alpha +2 ) } \biggr] \biggl[ \frac{1}{2}- \frac{1}{ ( \beta +1 ) ( \beta +2 ) } \biggr] \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} ( \eta _{1},\vartheta _{1} ) \biggr\vert ^{q} \\& \qquad {}+ \biggl[ \frac{3}{2}- \frac{2\alpha +3}{ ( \alpha +1 ) ( \alpha +2 ) } \biggr] \biggl[ \frac{3}{2}- \frac{2\beta +3}{ ( \beta +1 ) ( \beta +2 ) } \biggr] \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} ( \eta _{1},\vartheta _{2} ) \biggr\vert ^{q} \\& \qquad {}+ \biggl[ \frac{1}{2}- \frac{1}{ ( \alpha +1 ) ( \alpha +2 ) } \biggr] \biggl[ \frac{1}{2}- \frac{1}{ ( \beta +1 ) ( \beta +2 ) } \biggr] \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} ( \eta _{2},\vartheta _{1} ) \biggr\vert ^{q} \\& \qquad {}+ \biggl[ \frac{1}{2}- \frac{1}{ ( \alpha +1 ) ( \alpha +2 ) } \biggr] \biggl[ \frac{3}{2}- \frac{2\beta +3}{ ( \beta +1 ) ( \beta +2 ) } \biggr] \biggl\vert \frac {\partial ^{2}f}{\partial t\partial s}(b,d) \biggr\vert ^{q} \biggr) ^{{\frac{1}{q}}} \\& \qquad {}+ \biggl( \biggl[ \frac{1}{2}- \frac{1}{ ( \alpha +1 ) ( \alpha +2 ) } \biggr] \biggl[ \frac{3}{2}- \frac{2\beta +3}{ ( \beta +1 ) ( \beta +2 ) } \biggr] \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} ( \eta _{1},\vartheta _{1} ) \biggr\vert ^{q} \\& \qquad {}+ \biggl[ \frac{1}{2}- \frac{1}{ ( \alpha +1 ) ( \alpha +2 ) } \biggr] \biggl[ \frac{1}{2}- \frac{1}{ ( \beta +1 ) ( \beta +2 ) } \biggr] \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} ( \eta _{1},\vartheta _{2} ) \biggr\vert ^{q} \\& \qquad {}+ \biggl[ \frac{3}{2}- \frac{2\alpha +3}{ ( \alpha +1 ) ( \alpha +2 ) } \biggr] \biggl[ \frac{3}{2}- \frac{2\beta +3}{ ( \beta +1 ) ( \beta +2 ) } \biggr] \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} ( \eta _{2},\vartheta _{1} ) \biggr\vert ^{q} \\& \qquad {}+ \biggl[ \frac{3}{2}- \frac{2\alpha +3}{ ( \alpha +1 ) ( \alpha +2 ) } \biggr] \biggl[ \frac{1}{2}- \frac{1}{ ( \beta +1 ) ( \beta +2 ) } \biggr] \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} ( \eta _{2},\vartheta _{2} ) \biggr\vert ^{q} \biggr) ^{{\frac{1}{q}}} \\& \qquad {}+ \biggl( \biggl[ \frac{1}{2}- \frac{1}{ ( \alpha +1 ) ( \alpha +2 ) } \biggr] \biggl[ \frac{1}{2}- \frac{1}{ ( \beta +1 ) ( \beta +2 ) } \biggr] \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} ( \eta _{1},\vartheta _{1} ) \biggr\vert ^{q} \\& \qquad {}+ \biggl[ \frac{1}{2}- \frac{1}{ ( \alpha +1 ) ( \alpha +2 ) } \biggr] \biggl[ \frac{3}{2}- \frac{2\beta +3}{ ( \beta +1 ) ( \beta +2 ) } \biggr] \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} ( \eta _{1},\vartheta _{2} ) \biggr\vert ^{q} \\& \qquad {}+ \biggl[ \frac{3}{2}- \frac{2\alpha +3}{ ( \alpha +1 ) ( \alpha +2 ) } \biggr] \biggl[ \frac{1}{2}- \frac{1}{ ( \beta +1 ) ( \beta +2 ) } \biggr] \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} ( \eta _{2},\vartheta _{1} ) \biggr\vert ^{q} \\& \qquad {}+ \biggl[ \frac{3}{2}- \frac{2\alpha +3}{ ( \alpha +1 ) ( \alpha +2 ) } \biggr] \biggl[ \frac{3}{2}- \frac{2\beta +3}{ ( \beta +1 ) ( \beta +2 ) } \biggr] \biggl\vert \frac {\partial ^{2}\xi }{\partial t\partial s} ( \eta _{2},\vartheta _{2} ) \biggr\vert ^{q} \biggr) ^{{{\frac{1}{q}}}}. \end{aligned}$$
(2.23)