Linear maps preserving the higher numerical ranges of tensor products of matrices
Ajda Fošner, Zejun Huang, Chi-Kwong Li, Yiu‐Tung Poon, Nung-Sing Sze · Linear and Multilinear Algebra · 2013
For a positive integer , let be the set of complex matrices. Suppose are positive integers and . Denote by the -numerical range of a matrix . It is shown that a linear map satisfiesfor all and if and only if there is a unitary such that one of the following holds. For all , , and for all , , where (1) is the identity map or the transposition map , or (2) and has the form or .