Some authors include an additional axiom referred to as the closure under the operation "⋅", which means that a ⋅ b is an element of G for every a and b in G. This condition is subsumed by requiring "⋅" to be a binary operation on G. See Lang 2002. Lang, Serge (2002), Algebra, Graduate Texts in Mathematics, vol. 211 (Revised third ed.), New York: Springer-Verlag, ISBN978-0-387-95385-4, MR1878556
Hall 1967, p. 1, §1.1: "The idea of a group is one which pervades the whole of mathematics both pure and applied." Hall, G. G. (1967), Applied Group Theory, American Elsevier Publishing Co., Inc., New York, MR0219593, an elementary introduction.
Coornaert, Delzant & Papadopoulos 1990. Coornaert, M.; Delzant, T.; Papadopoulos, A. (1990), Géométrie et théorie des groupes [Geometry and Group Theory], Lecture Notes in Mathematics (in French), vol. 1441, Berlin, New York: Springer-Verlag, ISBN978-3-540-52977-4, MR1075994.
Rosen 2000, p. 54, (Theorem 2.1). Rosen, Kenneth H. (2000), Elementary Number Theory and its Applications (4th ed.), Addison-Wesley, ISBN978-0-201-87073-2, MR1739433.
Solomon 2018. Solomon, Ronald (2018), "The classification of finite simple groups: A progress report", Notices of the AMS, 65 (6): 1, doi:10.1090/noti1689
The word homomorphism derives from Greek ὁμός—the same and μορφή—structure. See Schwartzman 1994, p. 108. Schwartzman, Steven (1994), The Words of Mathematics: An Etymological Dictionary of Mathematical Terms Used in English, Mathematical Association of America, ISBN978-0-88385-511-9.