1. R. E. Hartwig, “How to partially order regular elements,” Math. Japon. 25(1), 1–13 (1980).
2. K. S. S. Nambooripad, “The natural partial order on a regular semigroup,” Proc. Edinb. Math. Soc. (2) 23(3), 249–260 (1980).
3. G. Frobenius, “Über die Darstellung der endlichen Gruppen durch lineare Substitutionen,” in Sitzungsberichte (Preuss. Akad. Wiss. Berlin, Berlin, 1897), pp. 994–1015.
4. S. Pierce, “General introduction,” in A Survey of Linear Preserver Problems, Linear Multilinear Algebra (Gordon and Breach Science Publ., Yverdon, 1992), Vol. 33, Nos. 1–2, pp. 3–5.
5. J. de Pillis, “Linear Transformations which preserve Hermitian and positive semidefinite operators,” Pacific J. Math. 23, 129–137 (1967).