1. On real eigenvalues of complex matrices;Carlson, David H.;Pacific J. Math.,1965
2. Separation of two linear subspaces;Davis, Chandler;Acta Sci. Math. (Szeged),1958
3. Products of symmetries;Halmos, Paul R.;Bull. Amer. Math. Soc.,1958
4. Products of self-adjoint operators;Radjavi, Heydar;Michigan Math. J.,1969