We study an inverse problem on the half-linear Dirichlet eigenvalue problem , where with and r is a positive function defined on . Using eigenvalues and nodal data (the lengths of two consecutive zeros of solutions), we reconstruct and its derivatives. Our method is based on (Law and Yang in Inverse Probl. 14:299-312, 779-780, 1998; Shen and Tsai in Inverse Probl. 11:1113-1123, 1995), and our result extends the result in (Shen and Tsai in Inverse Probl. 11:1113-1123, 1995) for the linear case to the half-linear case.
MSC: 34A55, 34B24, 47A75.
The subject under investigation is the half-linear eigenvalue problem consisting of
where with , and r is a positive function defined on . By [1-4], it is well known that the problem (1) has countably many eigenpairs , and the eigenfunction has exactly nodal points in (0,1), say . In this paper, we intend to give the representation of the function and its derivatives in (1) by using eigenvalues and nodal points. This formation is treated as the reconstruction formula. Such a problem is called an inverse nodal problem and has attracted researchers’ attention. Readers can refer to [5-7] for the linear case (), and to [8,9] for the general case ().
are considered. The authors in  studied (2) with Dirichlet boundary conditions
while the authors in  studied (2) with eigenparameter dependent boundary conditions
Both of them first used the modified Prüfer substitution to derive the asymptotic expansion of eigenvalues and nodal points and then gave the reconstruction formula of by using the nodal data. Note that the authors did not deal with the derivatives of in [8,9]. Besides, the author in  considered the same issue for the p-Laplacian energy-dependent Sturm-Liouville problem,
coupling with the Dirichlet boundary conditions.
In , Shen and Tsai studied the inverse nodal problem on the string equation with Dirichlet boundary conditions. They employed the standard difference operator △ to give a reconstruction formulas for and its derivatives. On the other hand, Law and Yang  not only studied the inverse nodal problems for the linear problem
with separated boundary conditions
where , and gave a reconstruction formulas for and its derivatives, but they also mentioned that the formulas for and its derivatives in the string equation with (3) are still valid. They applied the difference quotient operator δ in the formulas.
The main aim and methods of this study are basically the same as the ones in [6,7]. Here we employ a modified Prüfer substitution on (1) derived by the generalized sine function . The well-known properties of can be referred to [1,2,4], etc. It shall be mentioned that is not at odd multiples of as , and not at even multiples of as . These lead to that the reconstruction formulas for Nth derivatives, , in [6,7] cannot be extended to the half-linear case () in this article.
Now, define the difference operator △ and the difference quotient operator δ as follows:
This paper is organized as follows. In Section 2, we give the asymptotic estimates for eigenvalues. This step makes us know such quantities well. It is necessary to specify the orders of the expansion terms in the proofs of the main results. In Section 3, the proofs of the main theorems are given.
2 Asymptotic estimates for eigenvalues
Before we prove the main results, we derive the eigenvalue expansion. Note that if , the last term in (6) can be replaced by (cf. [, p.171]). In [, Theorem 2.5], they proved the error term is if . The smoothness of increases, and the smaller error can be derived.
Then a direct calculation yields
Let . Note that if is valid in some subinterval of , the term will be constant in this subinterval. This implies that the function depends on in this subinterval from (8). This will contradict our original problem. Hence, the points satisfying shall be isolated. Then, in (8)
for sufficiently large n. Substituting (11)-(12) into (10), the eigenvalue estimates (6) can be derived. □
3 Proofs of Theorems 1-2
On the other hand, by the mean value theorem and (14), one also has
Before we prove Theorem 2, one has to derive the asymptotic behavior of at the nodal points. Note that the series in (11) is uniformly convergent on . Set . Integrating (8) over and applying (11), one has
By the Taylor expansion theorem and integration by parts, we find
For convenience, we denote
Remark 1 Note that the function is defined by the integral of . By the unlike property of , is not a smooth function. This is the main reason that the result in [, Theorem 5.1] does not hold in our case, .
Proof The first part of the proof is followed by the mean value theorem and the asymptotic estimates for nodal length (14), i.e.,
The second part is similar to the one of the first part. So it is omitted here. □
The following corollary is similar to [, Lemma 2.3]. We give the proof for the convenience of the readers.
Proof of Theorem 2 Recall (19). By Theorem 3 and Lemma 2, one has and for . By Theorem 3 and Corollary 1, one can obtain for . By Theorem 3, Lemma 1 and the definition of , one has and are for . Hence, one can find
Successively, we have
Therefore, substituting (20) into (23)-(24), this completes the proof. □
The authors declare that they have no competing interests.
The author WCW was a major contributor in writing the manuscript. The author YHC performed the literature review. Both of them performed the final editing of the manuscript. They also gave their final approval of the version to be submitted and any revised version.
The authors express their thanks to the referees for helpful comments. The first author is supported in part by the National Science Council, Taiwan under contract number NSC 102-2115-M-507-001. The author YH Cheng is supported by the National Science Council, Taiwan under contract numbers NSC 102-2115-M-152 -002.
Eberhart, W, Elbert, A: On the eigenvalues of a half-linear boundary value problem. Math. Nachr.. 213, 57–76 (2000). Publisher Full Text
Reichel, W, Walter, W: Sturm-Liouville type problems for the p-Laplacian under asymptotic non-resonance conditions. J. Differ. Equ.. 156, 50–70 (1999). Publisher Full Text
McLaughlin, JR: Inverse spectral theory using nodal points as data - a uniqueness result. J. Differ. Equ.. 73, 354–362 (1988). Publisher Full Text
Shen, CL, Tsai, TM: On a uniform approximation of the density function of a string equation using eigenvalues and nodal points and some related inverse nodal problems. Inverse Probl.. 11, 1113–1123 (1995). Publisher Full Text
Addendum 14, 779-780 (1998)Publisher Full Text
Law, CK, Lian, WC, Wang, WC: Inverse nodal problem and Ambarzumyan problem for the p-Laplacian. Proc. R. Soc. Edinb. A. 139, 1261–1273 (2009). Publisher Full Text
Wang, WC, Cheng, YH, Lian, WC: Inverse nodal problems for the p-Laplacian with eigenparameter dependent boundary conditions. Math. Comput. Model.. 54, 2718–2724 (2011). Publisher Full Text
Li, HJ, Yeh, CC: Sturmian comparison theorem for half-linear second-order differential equations. Proc. R. Soc. Edinb. A. 125, 1193–1204 (1995). Publisher Full Text
Walter, W: Sturm-Liouville theory for the radial -operator. Math. Z.. 227, 175–185 (1998). Publisher Full Text