Abstract
Inspired by the condition \(\sup_{\lambda\in\mathbb{C}}||e^{\lambda Y}Xe^{-\lambda Y}||<\infty\) , which is equivalent to the commutativity of two operators \(X,Y\in\mathcal{B}(\mathcal{H})\) , we establish that for a state \(\varphi\) on a von Neumann algebra \(\mathcal{M}\) the following conditions are equivalent: (i) \(\varphi\) is tracial; (ii) \(\sup_{\lambda\in\mathbb{C}}|\varphi(e^{\lambda Y}Xe^{-\lambda Y})|<\infty\) for all positive operators \(X,Y\in\mathcal{M}\) ; (iii) \(|\varphi(\textrm{Re}(X^{2})|\leq\varphi(X^{*}X)\) for all \(X\in\mathcal{M}\) ; (iv) \(\varphi(X^{*}Y+Y^{*}X)=\varphi(XY^{*}+YX^{*})\) for all unitary operators \(X,Y\in\mathcal{M}\) . We also provide new criteria for the commutativity of von Neumann algebras.