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On uniqueness of recovering the convolution integro-differential operator from the spectrum of its non-smooth one-dimensional perturbation
- S. A. Buterin^{1}Email authorView ORCID ID profile and
- S. V. Vasiliev^{1}
- Received: 24 October 2017
- Accepted: 5 April 2018
- Published: 13 April 2018
Abstract
We consider a one-dimensional perturbation of the convolution integro-differential operator of arbitrary order on a finite interval. The inverse problem of recovering the convolution component from the spectrum is studied, provided that the perturbation term is known a priori. We prove the uniqueness theorem for this inverse problem and establish the minimal requirements on the perturbation term guarantying the informativity of the spectrum.
Keywords
- Integro-differential operator
- One-dimensional perturbation
- Convolution
- Inverse spectral problem
MSC
- 34A55
- 45J05
- 47G20
1 Introduction
Inverse problems of spectral analysis consist in recovering operators from their spectral characteristics. Such problems often appear in mathematics, mechanics, physics, electronics, geophysics, meteorology, and other branches of natural sciences and engineering. The greatest success in the inverse problem theory has been achieved for the Sturm–Liouville operator (see [1–4] and the references therein) and afterwards for higher-order differential operators [5–7]. For integro-differential and other classes of non-local operators, the classical methods of inverse spectral theory either do not work or require essential modifications. Inverse problems for some integro-differential operators were studied in [8–31] and other papers.
Various aspects of recovering the functions \(g(x)\) and \(v(x)\) from spectral data of the eigenvalue problem \({\mathcal {A}}\) in the cases \(n=1\) and \(n=2\), provided that the function \(M(x)\) is known a priori and \(a=0\), were studied in [9, 17, 20, 22, 24]. Moreover, in [19] this inverse problem was studied in the self-adjoint case when \(n=2\), \(M(x)=0\), \(g(x)=\pm \overline{v(x)}\) and instead of (2) the Dirichlet boundary conditions \(y(0)=y(\pi)=0\) were imposed.
In the present paper we study the following conversed inverse problem.
Inverse problem 1
Given the spectrum \(\{\lambda_{k}\}_{k\ge 1}\) of the problem \({\mathcal {A}}\), find the function \(M(x)\), provided that the functions \(g(x)\) and \(v(x)\) are known a priori.
A nonlinear inclusion of the function \(M(x)\) into the representation of the characteristic function for the boundary value problem \({\mathcal {A}}\) (see (6) below) makes recovering the function \(M(x)\) more difficult than recovering \(g(x)\) and \(v(x)\). In [13] inverse problem 1 was studied in the case when the functions \(g(x)\), \(v(x)\) belong to \(W_{2}^{1}[0,\pi ]\) and satisfy the condition \(g(0)v(\pi) \ne 0\). The inverse problem was reduced to a nonlinear integral equation with singularity, whose solvability has been proven in the appropriate class of functions. This allowed to prove the uniqueness theorem and to obtain a constructive procedure for solving the inverse problem along with necessary and sufficient conditions of its solvability. However, in the case \(g(x), v(x)\in L_{2}(0,\pi)\), it is not possible to derive such an equation. In the present paper, using another approach, we prove the following uniqueness theorem.
Theorem 1
- (i)
Let \(n=1\) or \(n=2\). Then the specification of the spectrum \(\{\lambda_{k}\}_{k\ge 1}\) uniquely determines the function \(M(x)\) a.e. on \((0,b-a)\).
- (ii)
Let \(n>2\). Then the specification of the spectrum \(\{\lambda_{k} \}_{k\ge 1}\) along with the number γ in representation (16) (see below) uniquely determines the function \(M(x)\) a.e. on \((0,b-a)\).
- (iii)
If \(g(x)=0\) a.e. on \((0,a)\) and \(v(x)=0\) a.e. on \((b,\pi)\), then for any \(n\ge 1\) the spectrum \(\{\lambda_{k}\}_{k\ge 1}\) does not depend on behavior of the function \(M(x)\) on the interval \((b-a,\pi)\).
Part (iii) of the theorem infers the minimality of the requirements put on \(g(x)\) and \(v(x)\). In part (i) the number γ mentioned in (ii) is determined by the specification of the spectrum. It is valid also for \(n>2\) under some additional conditions on \(g(x)\) and \(v(x)\). For example, in [13] this was established in the case \(g(x), v(x)\in W_{2}^{1}[0,\pi ]\) when \(g(0)v(\pi)\ne 0\). The following uniqueness theorem uses the generalization of this condition.
Theorem 2
We also note that in the case of \(n=1,2\), \(g(x)\equiv 0\) and appropriate boundary conditions, various aspects of recovering the function \(M(x)\) from the spectrum were studied in [8, 9, 11, 12, 14, 16, 18, 21, 22, 24–29] and other works. In [30] and [31] the case of fractional order \(n\notin {\mathbb{N}}\) was studied (see also [32, 33] for fractional integro-differential operators and singular integral operators).
The paper is organized as follows. In the next section we introduce and investigate the characteristic function \(\Delta (\lambda)\) of the boundary value problem \({\mathcal {A}}\). In Sect. 3 we study some aspects of recovering \(\Delta (\lambda)\) from its zeros. Theorems 1 and 2 are proven in Sect. 4.
2 Preliminary information
Lemma 1
Eigenvalues of the problem \({\mathcal {A}}\) coincide with zeros of its characteristic function.
Proof
Let \(\Delta (\lambda_{0})=0\). Then the function \(g_{n}(x,\lambda_{0})\) is, obviously, a solution of the boundary value problem \({\mathcal {A}}\). Since \(g(x)\not\equiv 0\), we have \(g_{n}(x,\lambda)\not\equiv 0\) for each λ and, hence, \(g_{n}(x,\lambda_{0})\) is an eigenfunction, while \(\lambda_{0}\) is the corresponding eigenvalue.
Lemma 2
Proof
Lemma 3
Proof
The following lemma gives another representation for the characteristic function.
Lemma 4
3 On determination of \(\Delta (\lambda)\) by the spectrum
In what follows, along with \({\mathcal {A}}\) we consider the boundary value problem \(\tilde{\mathcal {A}}={\mathcal {A}}(n,\tilde{M},v,g)\) of the same order n and with the same functions \(g(x)\) and \(v(x)\). We agree that if a certain symbol α denotes an object related to \({\mathcal {A}}\), then this object with tilde α̃ denotes the analogous object corresponding to \(\tilde{\mathcal {A}}\), and \(\hat{\alpha }=\alpha - \tilde{\alpha }\).
Lemma 5
Let \(n\in \{1,2\}\). If \(\{\lambda_{k}\}_{k\ge 1}= \{\tilde{\lambda }_{k}\}_{k\ge 1}\), then \(\Delta (\lambda)\equiv \tilde{\Delta }(\lambda)\). In other words, the specification of the spectrum uniquely determines the characteristic function.
Proof
Lemma 6
Let \(n>2\). If the functions \(g(x)\) and \(v(x)\) satisfy the hypothesis of Theorem 2, then the specification of the spectrum uniquely determines γ in representation (16).
Proof
4 Proof of Theorem 1 and Theorem 2
Lemma 7
Proof
Lemma 8
Proof
Further, if \(\gamma =\tilde{\gamma }\) and \(\{\lambda_{k}\}_{k\ge 1}= \{\tilde{\lambda }_{k}\}_{k\ge 1}\), then formula (16) implies \(\Delta (\lambda)\equiv \tilde{\Delta }(\lambda)\). By virtue of (14), this infers \(\mu (x)=\tilde{\mu }(x)\) and, hence, (31) gives \(\Delta_{1}(\lambda)\equiv \tilde{\Delta }_{1}(\lambda)\). According to part (i) of Theorem 1 for \(n=1\), we get \(M_{1}(x)= \tilde{M}_{1}(x)\) a.e. on \((0,b-a)\) and, by virtue of (11), \(M(x)=\tilde{M}(x)\) a.e. on \((0,b-a)\). Thus, part (i) for \(n=2\) and part (ii) of Theorem 1 are proven.
We actually have established that \(\Delta (\lambda)\equiv \tilde{\Delta }(\lambda)\) if and only if \(\hat{M}(x)=0\) a.e. on \((0,b-a)\), which proves part (iii) of Theorem 1.
For the proof of Theorem 2, it remains to note that under its hypothesis, according to Lemma 6, the specification of the spectrum uniquely determines γ.
5 Conclusions
Studying inverse problems for one-dimensional perturbations of integro-differential and integral operators is a prospective direction in the inverse spectral theory because many important operators can be represented in this form. For example, in [36] it was shown how the uniqueness theorem for the classical inverse Sturm–Liouville problem follows from a uniqueness theorem for the one-dimensional perturbation of a Volterra integral operator.
Declarations
Acknowledgements
This work was supported by Grant 17-11-01193 of the Russian Science Foundation.
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All authors contributed equally and read and approved the final version of the manuscript.
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Competing interests
The authors declare that they have no competing interests.
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