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
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 . 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].
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 .
We consider the singular Sturm-Liouville problem
in which the function , A, H are finite numbers and . Next, we denote by the solution of (1.1) satisfying the initial condition
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:
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:
Lemma 2.3Assume that . Then, as ,
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
The nodal length is
This completes the proof of Lemma 2.3. □
Lemma 2.4Suppose . Then, for almost every with ,
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 function satisfies
for almost every with . We note that the asymptotic expression for in Theorem 2.1 implies that .
Proof When we consider (2.4) in the form
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 sequence converges to , then, for any large enoughn, with as
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 as with ,
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 2.2 converges 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
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,
Then, we may apply the Lebesque dominated convergence theorem to show that (2.5) is valid. The proof of Theorem 2.2 is completed. □
The authors declare that they have no competing interests.
MS wrote the first draft and ESP corrected and improved the final version. Both authors read and approved the final draft.
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.
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