Cotlar–Stein lemma

The Cotlar–Stein lemma is a result in functional analysis, particularly in the theory of bounded operators on Hilbert spaces. It provides a criterion under which a certain type of operator can be shown to be compact. While the lemma itself can be quite specialized, its essence can be articulated as follows: Suppose \(T\) is a bounded linear operator on a Hilbert space \(H\).