The Best Constant of Sobolev Inequality Corresponding to Clamped Boundary Value Problem

  • Kohtaro Watanabe1Email author,

    Affiliated with

    • Yoshinori Kametaka2,

      Affiliated with

      • Hiroyuki Yamagishi3,

        Affiliated with

        • Atsushi Nagai4 and

          Affiliated with

          • Kazuo Takemura4

            Affiliated with

            Boundary Value Problems20112011:875057

            DOI: 10.1186/1687-2770-2011-875057

            Received: 14 August 2010

            Accepted: 10 February 2011

            Published: 7 March 2011


            Green's function of the clamped boundary value problem for the differential operator on the interval is obtained. The best constant of corresponding Sobolev inequality is given by . In addition, it is shown that a reverse of the Sobolev best constant is the one which appears in the generalized Lyapunov inequality by Das and Vatsala (1975).

            1. Introduction

            For , , let be a Sobolev (Hilbert) space associated with the inner product :
            The fact that induces the equivalent norm to the standard norm of the Sobolev (Hilbert) space of th order follows from Poincaré inequality. Let us introduce the functional as follows:
            To obtain the supremum of (i.e., the best constant of Sobolev inequality), we consider the following clamped boundary value problem:
            Concerning the uniqueness and existence of the solution to , we have the following proposition. The result is expressed by the monomial :

            Proposition 1.1.

            For any bounded continuous function on an interval , has a unique classical solution expressed by
            where Green's function is given by

   is the determinant of matrix , , and .

            With the aid of Proposition 1.1, we obtain the following theorem. The proof of Proposition 1.1 is shown in Appendices A and B.

            Theorem 1.2.

            (i) The supremum (abbreviated as if there is no confusion) of the Sobolev functional is given by

            (ii) is attained by , that is, .

            Clearly, Theorem 1.2(i), (ii) is rewritten equivalently as follows.

            Corollary 1.3.

            Let , then the best constant of Sobolev inequality (corresponding to the embedding of into )

            is . Moreover the best constant is attained by , where is an arbitrary complex number.

            Next, we introduce a connection between the best constant of Sobolev- and Lyapunov-type inequalities. Let us consider the second-order differential equation
            where . If the above equation has two points and in satisfying , then these points are said to be conjugate. It is wellknown that if there exists a pair of conjugate points in , then the classical Lyapunov inequality
            holds, where . Various extensions and improvements for the above result have been attempted; see, for example, Ha [1], Yang [2], and references there in. Among these extensions, Levin [3] and Das and Vatsala [4] extended the result for higher order equation
            For this case, we again call two distinct points and s2conjugate if there exists a nontrivial solution of (1.12) satisfying

            We point out that the constant which appears in the generalized Lyapunov inequality by Levin [3] and Das and Vatsala [4] is the reverse of the Sobolev best embedding constant.

            Corollary 1.4.

            If there exists a pair of conjugate points on with respect to (1.12), then

            where is the best constant of the Sobolev inequality (1.9).

            Without introducing auxiliary equation and the existence result of conjugate points as [2, 4], we can prove this corollary directly through the Sobolev inequality (the idea of the proof origins to Brown and Hinton [5, page 5]).

            Proof of Corollary 1.4.

            In the second inequality, the equality holds for the function which attains the Sobolev best constant, so especially it is not a constant function. Thus, for this function, the first inequality is strict, and hence we obtain

            we obtain the result.

            Here, at the end of this section, we would like to mention some remarks about (1.12). The generalized Lyapunov inequality of the form (1.14) was firstly obtained by Levin [3] without proof; see Section 4 of Reid [6]. Later, Das and Vatsala [4] obtained the same inequality (1.14) by constructing Green's function for . The expression of the Green's function of Proposition 1.1 is different from that of [4]. The expression of [4, Theorem 2.1] is given by some finite series of and on the other hand, the expression of Proposition 1.1 is by the determinant. This complements the results of [79], where the concrete expressions of Green's functions for the equation but different boundary conditions are given, and all of them are expressed by determinants of certain matrices as Proposition 1.1.

            2. Reproducing Kernel

            First we enumerate the properties of Green's function of . has the following properties.

            Lemma 2.1.

            Consider the following:



            For and , , we have from (1.5)
            For , noting the fact , we have (1). Next, for and , we have from (2.5)
            Since , we have
            Note that subtracting the th row from th row, the second equality holds. Equation is shown by the same way. Hence, we have (2). For , we have

            where we used the fact , . So we have (3), and (4) follows from (3).

            Using Lemma 2.1, we prove that the functional space associated with inner norm is a reproducing kernel Hilbert space.

            Lemma 2.2.

            For any , one has the reproducing property


            For functions and with arbitrarily fixed in , we have
            Integrating this with respect to on intervals and , we have

            Using (1), (2), and (4) in Lemma 2.1, we have (2.9).

            3. Sobolev Inequality

            In this section, we give a proof of Theorem 1.2 and Corollary 1.3.

            Proof of Theorem 1.2 and Corollary 1.3.

            Applying Schwarz inequality to (2.9), we have
            Note that the last equality holds from (2.9); that is, substituting (2.9), . Let us assume that
            holds (this will be proved in the next section). From definition of , we have
            Substituting in to the above inequality, we have
            Combining this and trivial inequality , we have
            Hence, we have

            which completes the proof of Theorem 1.2 and Corollary 1.3.

            Thus, all we have to do is to prove (3.2).

            4. Diagonal Value of Green's Function

            In this section, we consider the diagonal value of Green's function, that is, . From Proposition 1.1, we have for

            Thus, we can expect that takes the form . Precisely, we have the following proposition.

            Proposition 4.1.


            where satisfy .

            To prove this proposition, we prepare the following two lemmas.

            Lemma 4.2.

            Let , where
            ( satisfy ), then it holds that

            Lemma 4.3.

            Let , where , then it holds that (4.6) and .

            Proof of Proposition 4.1.

            From Lemmas 4.2 and 4.3, and satisfy BVP (in case of ). So we have

            Inserting (4.9) into (4.8), we have Proposition 4.1.

            Proof of Lemma 4.2.

            then differentiating times we have
            At first, for , we have
            The first term vanishes because
            The third term also vanishes because
            Thus, we have
            Hence, we have
            by which we obtain (4.5). Next, for , we have
            Since , we have . Thus, we have . For , we have
            The first term vanishes because . Next, we show that the second term also vanishes. Let
            Since , two rows, including the last row, coincide, and hence we have . Thus, we have . So we have obtained . By the same argument, we have . Hence, we have (4.6). Finally, we will show (4.7). For , noting , we have
            Thus, we obtain . Hence we have
            that is,

            This completes the proof of Lemma 4.2.

            Proof of Lemma 4.3.

            Differentiating times, we have
            For , noting , , and , we have
            Thus, we have (4.5). If , then we have
            Since , we have . Hence, we have (4.6). If , then we have

            This proves Lemma 4.3.


            Authors’ Affiliations

            Department of Computer Science, National Defense Academy
            Division of Mathematical Sciences, Graduate School of Engineering Science, Osaka University
            Tokyo Metropolitan College of Industrial Technology
            Department of Liberal Arts and Basic Sciences, College of Industrial Technology, Nihon University


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