Malnormal matrices
Abstract
We exhibit an operator norm bounded, infinite sequence \An\ of 3n × 3n complex matrices for which the commutator map X XAn - AnX 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 problems related to such matrices and provide numerical evidence.
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