(FUNDAMENTAL AND APPLIED MATHEMATICS)

1999, VOLUME 5, NUMBER 2, PAGES 627-635

## On the existence of invariant subspaces of dissipative operators in space with indefinite metric

A. A. Shkalikov

Abstract

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Let $\mathcal H$ be Hilbert space with fundamental symmetry $J=P$+-P-, where $P$± are mutualy orthogonal projectors such that $J2$ is identity operator. The main result of the paper is the following: if $A$ is a maximal dissipative operator in the Krein space $\mathcal K=\\left\{\mathcal H,J\\right\}$, the domain of $A$ contains $P_+\left(\mathcal H\right)$, and the operator $P$+AP- is compact, then there exists an $A$-invariant maximal non-negative subspace $\mathcal L$ such that the spectrum of the restriction $A|_\left\{\mathcal L\right\}$ lies in the closed upper-half complex plain.

This theorem is a modification of well-known results of L. S. Pontrjagin, H. Langer, M. G. Krein and T. Ja. Azizov. A new proof is proposed in this paper.

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