Periodic Potential From the Spectral Invariants

In this chapter, we consider the inverse problemInverse problem of the three-dimensional Schrödinger operatorSchrödinger operator L(q) with a periodic, relative to a lattice \(\Omega \) of \(\mathbb {R}^{3},\) potential q. Firstly, we construct a set D of trigonometric polynomials such that: a) D is dense in \(W_{2}^{s}(\mathbb {R}^{3}/\Omega ),\) where \(s>3,\) in the \(\mathbb {C}^{\infty }\) -topology, b) any element q of the set D can be determined constructively and uniquely, modulo inversion \(x\rightarrow -x\ \) and translations \(x\rightarrow x+\tau \) for \(\tau \in \mathbb {R}^{3}\) , from the given spectral invariantsSpectral invariants that were determined constructively from the given Bloch eigenvalues. Then a special class V of the periodic potentials is constructed, which can be easily and constructively determined from the spectral invariantsSpectral invariants. This chapter consists of seven sections. The first section is introduction, where we describe briefly the scheme of this chapter and discuss the related papers. In the second section using the spectral invariantsSpectral invariants obtained in Chap. 4 , we find the simplest invariants for the sets D and V. In the third, fourth, and fifth sections, we give algorithms for the unique determination of the potential \(q\in D\) and \(q\in V\) , respectively, from the simplest spectral invariants. In the sixth section, we consider the stability of the algorithm for \(q\in V\) with respect to the spectral invariantsSpectral invariants and Bloch eigenvalues. Finally, in the seventh section, we prove that there are no other periodic potentials in the set of a large class of functions whose Bloch eigenvalues coincide with the Bloch eigenvalues of \(q\in V.\) Thus, Chap. 5 gives some examples and ideas for finding the potential from the spectral invariantsSpectral invariants and hence from the Bloch eigenvalues. Besides, it gives a theoretical base ( a lot of nonlinear equations with respect to the Fourier coefficientsFourier coefficients of q) to solve numerically this problem.