Structure-preserving map between two algebraic structures of the same type
Not to be confused with holomorphism or homeomorphism.
In algebra, a homomorphism is a structure-preserving map between two algebraic structures of the same type (such as two groups, two rings, or two vector spaces). The word homomorphism comes from the Ancient Greek language: ὁμός (homos) meaning "same" and μορφή (morphe) meaning "form" or "shape". However, the word was apparently introduced to mathematics due to a (mis)translation of German ähnlich meaning "similar" to ὁμός meaning "same".[1] The term "homomorphism" appeared as early as 1892, when it was attributed to the German mathematician Felix Klein (1849–1925).[2]
Homomorphisms of vector spaces are also called linear maps, and their study is the subject of linear algebra.
The concept of homomorphism has been generalized, under the name of morphism, to many other structures that either do not have an underlying set, or are not algebraic. This generalization is the starting point of category theory.
A homomorphism may also be an isomorphism, an endomorphism, an automorphism, etc. (see below). Each of those can be defined in a way that may be generalized to any class of morphisms.
^Fricke, Robert (1897–1912). Vorlesungen über die Theorie der automorphen Functionen. B.G. Teubner. OCLC 29857037.
^See:
Ritter, Ernst (1892). "Die eindeutigen automorphen Formen vom Geschlecht Null, eine Revision und Erweiterung der Poincaré'schen Sätze" [The unique automorphic forms of genus zero, a revision and extension of Poincaré's theorem]. Mathematische Annalen (in German). 41: 1–82. doi:10.1007/BF01443449. S2CID 121524108. From footnote on p. 22: "Ich will nach einem Vorschlage von Hrn. Prof. Klein statt der umständlichen und nicht immer ausreichenden Bezeichnungen: "holoedrisch, bezw. hemiedrisch u.s.w. isomorph" die Benennung "isomorph" auf den Fall des holoedrischen Isomorphismus zweier Gruppen einschränken, sonst aber von "Homomorphismus" sprechen, … " (Following a suggestion of Prof. Klein, instead of the cumbersome and not always satisfactory designations "holohedric, or hemihedric, etc. isomorphic", I will limit the denomination "isomorphic" to the case of a holohedric isomorphism of two groups; otherwise, however, [I will] speak of a "homomorphism", … )
Fricke, Robert (1892). "Ueber den arithmetischen Charakter der zu den Verzweigungen (2,3,7) und (2,4,7) gehörenden Dreiecksfunctionen" [On the arithmetic character of the triangle functions belonging to the branch points (2,3,7) and (2,4,7)]. Mathematische Annalen (in German). 41 (3): 443–468. doi:10.1007/BF01443421. S2CID 120022176. From p. 466: "Hierdurch ist, wie man sofort überblickt, eine homomorphe*) Beziehung der Gruppe Γ(63) auf die Gruppe der mod. n incongruenten Substitutionen mit rationalen ganzen Coefficienten der Determinante 1 begründet." (Thus, as one immediately sees, a homomorphic relation of the group Γ(63) is based on the group of modulo n incongruent substitutions with rational whole coefficients of the determinant 1.) From footnote on p. 466: "*) Im Anschluss an einen von Hrn. Klein bei seinen neueren Vorlesungen eingeführten Brauch schreibe ich an Stelle der bisherigen Bezeichnung "meroedrischer Isomorphismus" die sinngemässere "Homomorphismus"." (Following a usage that has been introduced by Mr. Klein during his more recent lectures, I write in place of the earlier designation "merohedral isomorphism" the more logical "homomorphism".)
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ideals as the main references. The three isomorphism theorems, called homomorphism theorem, and two laws of isomorphism when applied to groups, appear explicitly...
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