J. Giribet, M. Langer, F. Martínez Pería, F. Philipp and C. Trunk,
Spectral enclosures for a class of block operator matrices,
J. Funct. Anal. 278 (2020), 108455, 30 pp.

Abstract:
We prove new spectral enclosures for the non-real spectrum of a class of 2×2 block operator matrices with self-adjoint operators A and D on the diagonal and operators B and -B* as off-diagonal entries. One of our main results resembles Gershgorin's circle theorem. The enclosures are applied to J-frame operators.

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