Malnormal matrices.

We exhibit an operator norm bounded, infinite sequence \{A_n\} of 3n \times 3n complex matrices for which the commutator map X\mapsto XA_n - A_nX is uniformly bounded below as an operator over the space of trace-zero self-adjoint matrices equipped with Hilbert–Schmidt norm. The construction is based on families of quantum expanders. We give several potential applications of these matrices to the study of quantum expanders. We formulate several natural conjectures and provide numerical evidence. [ABSTRACT FROM AUTHOR]