Comment on `The tan θ theorem with relaxed conditions', by Y. Nakatsukasa

Abstract

We show that in case of the spectral norm, one of the main results of the recent paper "The tan θ theorem with relaxed conditions", by Yuji Nakatsukasa, published in Linear Algebra and its Applications is a corollary of the tan θ theorem proven in [V.Kostrykin, K.A.Makarov, and A.K.Motovilov, On the existence of solutions to the operator Riccati equation and the tan θ theorem, IEOT 51 (2005), 121-140]. We also give an alternative finite-dimensional matrix formulation of another tan θ theorem proven in [S.Albeverio and A.K.Motovilov, The a priori tan θ theorem for spectral subspaces, IEOT 73 (2012), 413-430].