Abstract
In this study, the inverse nodal problem is solved for pLaplacian Schrödinger equation with energydependent potential function with the Dirichlet conditions. Asymptotic estimates of eigenvalues, nodal points and nodal lengths are given by using Prüfer substitution. Especially, an explicit formula for a potential function is given by using nodal lengths. Results are more general than the classical pLaplacian SturmLiouville problem. For the proofs, methods previously developed by Law et al. and Wang et al., in 2009 and 2011, respectively, are used. In there, they solved an inverse nodal problem for the classical pLaplacian SturmLiouville equation with eigenparameter boundary conditions.
MSC: 34A55, 34L20.
Keywords:
Prüfer substitution; inverse nodal problem; pLaplacian equation1 Introduction
Consider the following pLaplacian eigenvalue problem for
with the boundary conditions
where is a realvalued function, is a constant, and λ is the spectral parameter [1]. Equation (1.1) is also known as a onedimensional pLaplacian eigenvalue equation. Note that when , equation (1.1) becomes a SturmLiouville equation as
and the inverse problem described in (1.1), (1.2) in the [18].
The determination of the form of a differential operator from spectral data associated with it has enjoyed close attention from a number of authors in recent years. One such operator is the SturmLiouville operator. In the typical formulation of the inverse SturmLiouville problem, one seeks to recover both q and constants by giving the eigenvalues with another piece of spectral data. These data can take several forms, leading to many versions of the problem. Especially, the recent interest is a study by Hald and McLaughlin [9,10] wherein the given spectral information consists of a set of nodal points of eigenfunctions for the SturmLiouville problems. These results were extended to the case of problems with eigenparameterdependent boundary conditions by Browne and Sleeman [11]. On the other hand, Law et al.[12], Law and Yang [13] solved the inverse nodal problem of determining the smoothness of the potential function q of the SturmLiouville problem by using nodal data. In the past few years, the inverse nodal problem of SturmLiouville problem has been investigated by several authors [11,1416].
The eigenvalues of this problem were given as [1]
where
and an associated eigenfunction is denoted by . and are periodic functions satisfying the identity
for arbitrary . These functions are known as generalized sine and cosine functions and for become sine and cosine[17].
Now, we present some further properties of for deriving more detailed asymptotic formulas. These formulas are crucial in the solution of our problem.
Lemma 1.1[1]
(b)
According to the SturmLiouville theory, the zero set of the eigenfunction corresponding to is called the nodal set and is defined as the nodal length of . Using the nodal data, some uniqueness results, reconstruction and stability of potential functions have been obtained by many authors [9,11,1416,18].
Consider the pLaplacian eigenvalue problem
with the Dirichlet conditions
or with the Neumann boundary conditions
where and are realvalued functions, is a constant, and λ is the spectral parameter.
In this paper, the function r is known a priori and we try to construct the unknown function q by the dense nodal points in the interval considered.
This equation is known as the diffusion equation or quadratic of differential pencil. Eigenvalue equation (1.6) is important for both classical and quantum mechanics. For example, such problems arise in solving the KleinGordon equations, which describe the motion of massless particles such as photons. SturmLiouville energydependent equations are also used for modelling vibrations of mechanical systems in viscous media (see [19]). We note that in this type of problem the spectral parameter λ is related to the energy of the system, and this motivates the terminology ‘energydependent’ used for the spectral problem of the form (1.6). Inverse problems of quadratic pencil have been solved by many authors in the references [15,16,18,2027].
As in the pLaplacian SturmLiouville problem, for , eigenvalues of the problem given by (1.3), (1.4) are of the form
and associated eigenfunctions are denoted by .
This paper is organized as follows. In Section 2, we give asymptotic formulas for eigenvalues, nodal points and nodal lengths. In Section 3, we give a reconstruction formula for differential pencil by using nodal parameters.
2 Asymptotic estimates of nodal parameters
In this section, we study the properties of eigenvalues of pLaplacian operator (1.3) with Dirichlet conditions (1.4). For this, we introduce Prüfer substitution. One may easily obtain similar results for Neumann problems.
We define a modified Prüfer substitution
or
Differentiating both sides of equation (2.2) with respect to x and applying Lemma 1.1, one obtains that
Theorem 2.1The eigenvaluesof the Dirichlet problem given in (1.3), (1.4) have the form
Proof For problem (1.3), (1.4), let , and . Firstly, we integrate both sides of (2.3) over the interval :
Using the identity
and Lemma 1.1(b), we get
Then, using integration by parts, we have
where
Similarly, one can show that
Inserting these values in (2.5) and after some straightforward computations, we obtain (2.4). □
Theorem 2.2For problem (1.3), (1.4), the nodal points expansion satisfies
Proof Let and integrating (2.3) from 0 to , we have
By using the estimates of eigenvalues as
we obtain
□
Proof For large , integrating (2.3) on and then
or
By Lemma 1.1 and a similar process to that used in Theorem 2.1, we obtain that
where and . Similarly, one can show that
Inserting this value in (2.7), we obtain
and by Theorem 2.1,
□
3 Reconstruction of a potential function in the differential pencil
In this section, we give an explicit formula for a potential function. The method used in the proof of the theorem is similar to that for classical SturmLiouville problems [1,8].
Theorem 3.1Let, and assumerthat on the intervalis given a priori. Then
Proof Applying the mean value theorem for integrals to (2.6), with fixed n, there exists , we obtain
or
Considering (2.6), we can write that for ,
Then
This completes the proof. □
Conclusion 3.2 In Theorem 2.1, Theorem 2.2, Theorem 2.3 and Theorem 3.1, taking , we obtain results of the SturmLiouville problem given in [12].
Conclusion 3.3 In Theorem 2.1, Theorem 2.2, Theorem 2.3 and Theorem 3.1, taking , we obtain the results of an inverse nodal problem for differential pencil [15].
Competing interests
The author declares that they have no competing interests.
Acknowledgements
The author would like to thank the referees for valuable comments and suggestions on improving this paper.
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