Krein-\v{S}mul'jan Theorem Revisited

Abstract

We present a generalization of Krein-\v{S}mul'jan theorem which involves several operators. Given bounded selfadjoint operators $A,B_1,\ldots,B_m$ acting on a Hilbert space $\mathcal{H}$, we provide sufficient conditions to determine whether there are $\lambda_1,\ldots,\lambda_m\in \mathbb{R}$ such that $A + \sum_{i=1}^m \lambda_i B_i$ is a positive semidefinite operator.