Functions of perturbed noncommuting self-adjoint operators

Abstract: We consider functions $f(A,B)$ of noncommuting self-adjoint operators $A$ and $B$ that can be defined in terms of double operator integrals. We prove that if $f$ belongs to the Besov class $B_{\be,1}^1(\R^2)$, then we have the following Lipschitz type estimate in the trace norm: $|f(A_1,B_1)-f(A_2,B_2)|{\bS_1}\le\const(|A_1-A_2|{\bS_1}+|B_1-B_2|{\bS_1})$. However, the condition $f\in B{\be,1}^1(\R^2)$ does not imply the Lipschitz type estimate in the operator norm.