Hilbert-Schmidt factorization of a trace-class operator
= Hilbert-Schmidt factorization of a trace-class operator
An operator is trace class exactly when it factors as $T=AB$ with $A$ and $B$ Hilbert--Schmidt. The factors can be chosen so that $\|T\|_1=\|A\|_{\mathrm{HS}}\|B\|_{\mathrm{HS}}$ by using the <polar decomposition of a bounded operator>.