Tensor-product operator
= Tensor-product operator
A tensor-product operator factors as $A\otimes B$ on a <tensor product> of two spaces. If both factors are <positive semidefinite operators>, their tensor-product operator is positive: a <tensor-product basis> of eigenvectors has eigenvalues $a_ib_j\geq0$. This positivity underlies the necessary direction of the <positive partial transpose criterion>.