# Reconstruction of potential function for Sturm-Liouville operator with Coulomb potential

Etibar S Panakhov1 and Murat Sat2*

### Author affiliations

1 Department of Mathematics, Firat University, Elazig, 23119, Turkey

2 Department of Mathematics, Erzincan University, Erzincan, 24100, Turkey

For all author emails, please log on.

Boundary Value Problems 2013, 2013:49  doi:10.1186/1687-2770-2013-49

 Received: 12 December 2012 Accepted: 21 February 2013 Published: 8 March 2013

This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

### Abstract

In this paper, we are concerned with an inverse problem for the Sturm-Liouville operator with Coulomb potential using a new kind of spectral data that is known as nodal points. We give a reconstruction of q as a limit of a sequence of functions whose nth term is dependent only on eigenvalue and its associated nodal data. It is mentioned that this method is based on the works of Law and Yang, but we have applied the method to the singular Sturm-Liouville problem.

MSC: 34L05, 45C05.

##### Keywords:
Coulomb potential; nodal point; reconstruction formula

### 1 Introduction

Inverse problems of spectral analysis imply the reconstruction of a linear operator from some or other of its spectral characteristics. Such characteristics are spectra (for different boundary conditions), normalizing constants, spectral functions, scattering data, etc. An early important result in this direction, which gave vital impetus for further development of inverse problem theory, was obtained in [1]. At present, inverse problems are studied for certain special classes of ordinary differential operators. Inverse problems from two spectra are the most simple in their formulation and well studied in [2,3]. An effective method of constructing a regular and singular Sturm-Liouville operator from a spectral function or from two spectra is given in [4-7].

We note that the details of the inverse problem for singular equations are given in the monographs [8-11] and references therein.

In some recent interesting works [12,13], Hald and McLaughlin and Browne and Sleeman have taken a new approach to inverse spectral theory for the Sturm-Liouville problem. The novelty of these works lies in the use of nodal points as the given spectral data. In recent years, inverse nodal problems have been studied by several authors [14-21]etc.

In this paper, we deal with an inverse nodal problem for the Sturm-Liouville operator with Coulomb potential. We have reconstructed the potential function q from the nodal points of eigenfunctions, provided q is smooth enough. The method is based on a series of works by Law and Yang [14,17].

Before giving the main results, we mention some physical properties of the Sturm-Liouville operator with Coulomb potential. Learning about the motion of electrons moving under the Coulomb potential is of significance in quantum theory. Solving these types of problems allows us to find energy levels not only for a hydrogen atom but also for single valence electron atoms such as sodium. For hydrogen atom, the Coulomb potential is given by , where r is the radius of the nucleus, e is electronic charge. According to this, we use the time-dependent Schrödinger equation

where Ψ is the wave function, ħ is Planck’s constant and m is the mass of electron. In this equation, if the Fourier transform is applied

it will convert to energy equation dependent on the situation as follows:

Therefore, energy equation in the field with the Coulomb potential becomes

If this hydrogen atom is substituted to other potential area, then the energy equation becomes

If we make the necessary transformation, then we can get a Sturm-Liouville equation with Coulomb potential

where λ is a parameter which corresponds to the energy [22].

We consider the singular Sturm-Liouville problem

(1.1)

(1.2)

(1.3)

in which the function , A, H are finite numbers and . Next, we denote by the solution of (1.1) satisfying the initial condition

(1.4)

Let be the nth eigenvalue and , be nodal points of the nth eigenfunction. Also, let be the ith nodal domain of the nth eigenfunction and let be the associated nodal length. We also define the function by .

### 2 Main results

In this section, we try to obtain some asymptotic results and a reconstruction formula for the potential q, which has been obtained as a solution of an inverse nodal problem.

Lemma 2.1The solution of problem (1.1)-(1.3) has the following form:

(2.1)

where.

Proof Because satisfies equation (1.1), we get

By integrating the first term twice on the right-hand side by parts and taking the conditions into account (1.2), we find that

where . □

Lemma 2.2The eigenvalues of problem (1.1)-1.3) are the roots of (1.3). This spectral characteristic satisfies the following asymptotic expression[23]:

(2.2)

where

Lemma 2.3Assume that. Then, as,

(2.3)

(2.4)

Proof By using some iterations and trigonometric calculations in (2.1), we obtain

If is equal to zero and is not close to zero, then

Now, we take and . Because Taylor’s expansion for the arctangent function is given by

for some integer i, then

Therefore

The nodal length is

This completes the proof of Lemma 2.3. □

Lemma 2.4Suppose. Then, for almost everywith,

Proof Since , almost everywhere. Thus, given any , when n is sufficiently large and for almost every ,

This proves Lemma 2.4. □

Theorem 2.1The potential functionsatisfies

for almost everywith. We note that the asymptotic expression forin Theorem 2.1 implies that.

Proof When we consider (2.4) in the form

so that

By Lemma 2.4

for almost every .

It remains to show that for almost every ,

tends to zero as . Take a sequence of continuous functions which converges to q in . Then has a subsequence converging to q almost everywhere in . We call this subsequence . Take any x such that converges to . Then for a given , we can fix a large k such that . Hence

By Lemma 2.3,

and so it tends to zero as . By Lemma 2.4, the first term satisfies, when n is sufficiently large,

On the other hand,

Because is continuous, this term is arbitrarily every . Hence we conclude that . This proves Theorem 2.1. □

Lemma 2.5We take a sequenceconverges to, then, for any large enoughn, withas

Proof By (2.4) and observation that the integral is constant on any nodal interval , we obtain

and for this term converges to zero. □

Lemma 2.6Suppose that, then aswith,

Proof Firstly, let us show that if q is continuous on , the result is satisfied. Let . By using the intermediate value theorem, there exists such that

If x is close enough to a, the difference can be arbitrarily small. Then, for all , when n is large enough, with we get

In the above process, we assume that . The estimate also holds if . Hence if , converges to uniformly on . Thus can be arbitrarily small. Because is dense in , for any , there exists a sequence convergent to q in . Hence, fix n sufficiently large,

From the above process and Lemma 2.5, when k is large enough, the first two terms are arbitrarily small. Hence, as ,

□

Theorem 2.2converges toqin.

Proof When we consider the value of , we obtain that

It suffices to show that as

By using (2.4) we have

Hence, we only need to prove that for

and

(2.5)

From Lemma 2.6, the first limit holds and the second limit also holds. On the other hand, the sequence of functions

converges to 0 for almost every . Furthermore,

and

Then, we may apply the Lebesque dominated convergence theorem to show that (2.5) is valid. The proof of Theorem 2.2 is completed. □

### Competing interests

The authors declare that they have no competing interests.

### Authors’ contributions

MS wrote the first draft and ESP corrected and improved the final version. Both authors read and approved the final draft.

### Acknowledgements

The authors would like to thank the editor and referees for their valuable comments and remarks which led to a great improvement of the article.

### References

1. Ambartsumyan, VA: Über eine frage der eigenwerttheorie. Z. Phys.. 53, 690–695 (1929). Publisher Full Text

2. Levitan, BM: On the determination of the Sturm-Liouville operator from one and two spectra. Math. USSR, Izv.. 12, 179–193 (1978). Publisher Full Text

3. Isaacson, EL, Trubowitz, E: The inverse Sturm-Liouville problem. I. Commun. Pure Appl. Math.. 36, 767–783 (1983). Publisher Full Text

4. Gelfand, IM, Levitan, BM: On the determination of a differential equation by its spectral function. Izv. Akad. Nauk SSSR, Ser. Mat.. 15, 309–360 (1951) Ams, 253-304 (1955)

5. Hochstadt, H: The inverse Sturm-Liouville problem. Commun. Pure Appl. Math.. 26, 715–729 (1973). Publisher Full Text

6. Pöschel, J, Trubowitz, E: Inverse Spectral Theory, Academic Press, Boston (1987)

7. Rundell, W, Sack, EP: Reconstruction of a radially symmetric potential from two spectral sequences. J. Math. Anal. Appl.. 264, 354–381 (2001). Publisher Full Text

8. Carlson, R: Borg-Levinson theorem for Bessel operator. Pac. J. Math.. 177, 1–26 (1997). Publisher Full Text

9. Chadan, K, Colton, D, Paivarinta, L, Rundell, W: An Introduction to Inverse Scattering and Inverse Spectral Problems, SIAM, Philadelphia (1997)

10. Panakhov, ES, Sat, M: On the determination of the singular Sturm-Liouville operator from two spectra. Comput. Model. Eng. Sci.. 84, 1–11 (2012)

11. Hald, OH: Discontinuous inverse eigenvalue problem. Commun. Pure Appl. Math.. 37, 539–577 (1984). Publisher Full Text

12. Browne, PJ, Sleeman, BD: Inverse nodal problems for Sturm-Liouville equation with eigenparameter dependent boundary conditions. Inverse Probl.. 12, 377–381 (1996). Publisher Full Text

13. Hald, OH, McLaughlin, JR: Solution of inverse nodal problems. Inverse Probl.. 5, 307–347 (1989). Publisher Full Text

14. Chen, YT, Cheng, YH, Law, CK, Tsa, J: Convergence of reconstruction formula for the potential function. Proc. Am. Math. Soc.. 130, 2319–2324 (2002). Publisher Full Text

15. Yang, FX: A solution of the inverse nodal problem. Inverse Probl.. 13, 203–213 (1997). Publisher Full Text

16. McLaughlin, JR: Inverse spectral theory using nodal points as a data - a uniqueness result. J. Differ. Equ.. 73, 354–362 (1988). Publisher Full Text

17. Law, CK, Shen, CL, Yang, CF: The inverse nodal problem on the smoothness of the potential function. Inverse Probl.. 15, 253–263 (1999). Publisher Full Text

18. Yurko, VA, Freiling, G: Inverse nodal problems for differential operators on graphs with a cycle. Tamkang J. Math.. 41, 15–24 (2010)

19. Yang, CF: Inverse nodal problems for the Sturm-Liouville operator with eigenparameter dependent boundary conditions. Oper. Matrices. 6(1), 63–77 (2012)

20. Koyunbakan, H, Panakhov, ES: A uniqueness theorem for inverse nodal problem. Inverse Probl. Sci. Eng.. 15, 517–524 (2007). Publisher Full Text

21. Koyunbakan, H: Reconstruction of potential function for diffusion operator. Numer. Funct. Anal. Optim.. 30, 111–120 (2009). Publisher Full Text

22. Blohincev, DI: Foundations of Quantum Mechanics, GITTL, Moscow (1949)

23. Amirov, RK, Çakmak, Y, Gulyaz, S: Boundary value problem for second order differential equations with Coulomb singularity on a finite interval. Indian J. Pure Appl. Math.. 37, 125–140 (2006)