## Functional model and spectral analysis of discrete singular Hamiltonian system.(English)Zbl 1480.47052

Summary: A space of boundary values is constructed for a minimal symmetric operator, generated by a discrete singular Hamiltonian system, acting in the Hilbert space $$\ell_{\mathbf{A}}^{2}(\mathbb{N}_{0}; E \oplus E)$$ ($$\mathbb{N}_{0} = \{ 0,1,2,\ldots \}$$, $$\dim E = m < \infty$$) with maximal deficiency indices $$(m,m)$$ (in limit-circle case). A description of all maximal dissipative, maximal accumulative, self-adjoint and other extensions of such a symmetric operator is given in terms of boundary conditions at infinity. We construct a self-adjoint dilation of a maximal dissipative operator and its incoming and outgoing spectral representations, which make it possible to determine the scattering matrix of the dilation. We establish a functional model of the dissipative operator and construct its characteristic function in terms of the scattering matrix of the dilation. Finally, we prove the theorem on completeness of the system of eigenvectors and associated vectors (or root vectors) of the dissipative operator.

### MSC:

 47B39 Linear difference operators 47B44 Linear accretive operators, dissipative operators, etc. 47A20 Dilations, extensions, compressions of linear operators 47A40 Scattering theory of linear operators 47A45 Canonical models for contractions and nonselfadjoint linear operators 47B25 Linear symmetric and selfadjoint operators (unbounded) 47A75 Eigenvalue problems for linear operators 39A70 Difference operators
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