Remarks on an operator Wielandt inequality

Abstract

Let $A$ be a positive operator on a Hilbert space $\mathcal{H}$ with $0<m\leq A\leq M$ and $X$ and $Y$ are two isometries on $\mathcal{H}$ such that $X^{*}Y=0$. For every 2-positive linear map $\Phi$, define $$\Gamma=\left(\Phi(X^{*}AY)\Phi(Y^{*}AY)^{-1}\Phi(Y^{*}AX)\right)^{p}\Phi(X^{*}AX)^{-p}, \, \, \, p>0.$$ We consider several upper bounds for $\frac{1}{2}|\Gamma+\Gamma^{*}|$. These bounds complement a recent result on operator Wielandt inequality.