Locus of non-real eigenvalues of a class of linear relations in a Krein space

Abstract

It is a classical result that, if a maximal symmetric operator $T$ in a Krein space $\mathcal{H}=\mathcal{H}^-[\oplus]\mathcal{H}^+$ has the property $\mathcal{H}^-\subseteq\mathcal{D}_T$, then the imaginary part of its eigenvalue $\lambda$ from upper or lower half-plane is bounded by $\lvert \mathrm{Im}\,\lambda\rvert\leq2\lVert TP^- \rVert$. We prove that in both half-planes $\lvert \mathrm{Im}\,\lambda\rvert$ never exceeds $t_0\lVert TP^- \rVert$ for some constant $t_0\approx1.84$. The result applies to a closed symmetric relation $T$ and carries on a suitable, most notably dissipative, extension.