Using a quasilinear representation for unitarily invariant norms, we prove a basic inequality: Let A = L X X M be positive semideenite, where X 2 M m;n. Then k jX j p k 2 kL p k kM p k for all p > 0 and all unitarily invariant norms k k. We show how several inequalities of Cauchy-Schwarz type follow from this bound and obtain a partial analog of our results for l p norms.
