文摘
A self-adjoint operator A in a Kre?n space (\({\mathcal{K}, [\cdot , \cdot]}\)) is called partially fundamentally reducible if there exist a fundamental decomposition \({\mathcal{K} = \mathcal{K}_{+}[\dot{+}]\mathcal{K}_{-}}\) (which does not reduce A) and densely defined symmetric operators S + and S ?/sub> in the Hilbert spaces (\({\mathcal{K}_+, [\cdot , \cdot]}\)) and \({(\mathcal{K}_-, -[\cdot , \cdot])}\), respectively, such that each S + and S ?/sub> has defect numbers (1, 1) and the operator A is a self-adjoint extension of \({S = S_{+} \oplus (-S_-)}\) in the Kre?n space \({(\mathcal{K}, [\cdot , \cdot])}\). The operator A is interpreted as a coupling of operators S + and ?em class="EmphasisTypeItalic">S ?/sub> relative to some boundary triples \({\big(\mathbb{C},\,\Gamma_0^+,\,\Gamma_1^+\big)}\) and \({\big(\mathbb{C},\,\Gamma_0^-,\,\Gamma_1^-\big)}\). Sufficient conditions for a nonnegative partially fundamentally reducible operator A to be similar to a self-adjoint operator in a Hilbert space are given in terms of the Weyl functions m + and m ?/sub> of S + and S ?/sub> relative to the boundary triples \({\big(\mathbb{C},\,\Gamma_0^+,\,\Gamma_1^+\big)}\) and \({\big(\mathbb{C},\,\Gamma_0^-,\Gamma_1^-\big)}\). Moreover, it is shown that under some asymptotic assumptions on m + and m ?/sub> all positive self-adjoint extensions of the operator S are similar to self-adjoint operators in a Hilbert space.