Curvature inequalities for operators in the Cowen-Douglas class of a planar domain

Abstract

Fix a bounded planar domain $Ω.$ If an operator $T,$ in the Cowen-Douglas class $B_1(Ω),$ admits the compact set $\barΩ$ as a spectral set, then the curvature inequality $\mathcal K_T(w) \leq - 4 π^2 S_Ω(w,w)^2,$ where $S_Ω$ is the Szego kernel of the domain $Ω,$ is evident. Except when $Ω$ is simply connected, the existence of an operator for which $\mathcal K_T(w) = 4 π^2 S_Ω(w,w)^2$ for all $w$ in $Ω$ is not known. However, one knows that if $w$ is a fixed but arbitrary point in $Ω,$ then there exists a bundle shift of rank $1,$ say $S,$ depending on this $w,$ such that $\mathcal K_{S^*}(w) = 4 π^2 S_Ω(w,w)^2.$ We prove that these {\em extremal} operators are uniquely determined: If $T_1$ and $T_2$ are two operators in $B_1(Ω)$ each of which is the adjoint of a rank $1$ bundle shift and $\mathcal{K}_{T_1}({w}) = -4π^2 S(w,w)^2 = \mathcal{K}_{T_2}(w)$ for a fixed $w$ in $Ω,$ then $T_1$ and $T_2$ are unitarily equivalent. A surprising consequence is that the adjoint of only some of the bundle shifts of rank $1$ occur as extremal operators in domains of connectivity greater than $1.$ These are described explicitly.