Comment on `The tan {\theta} theorem with relaxed conditions', by Y.   Nakatsukasa
  We show that in case of the spectral norm, one of the main results of the recent paper "The tan {\theta} theorem with relaxed conditions", by Yuji Nakatsukasa, published in Linear Algebra and its Applications is a corollary of the tan {\theta} 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 {\theta} theorem, IEOT 51 (2005), 121-140]. We also give an alternative finite-dimensional matrix formulation of another tan {\theta} theorem proven in [S.Albeverio and A.K.Motovilov, The a priori tan {\theta} theorem for spectral subspaces, IEOT 73 (2012), 413-430]. 
Alexander K. Motovilov
math.SP math.NA physics.comp-ph