Character (mathematics)

In mathematics, a character is (most commonly) a special kind of function from a group to a field (such as the complex numbers). There are at least two distinct, but overlapping meanings.[1] Other uses of the word "character" are almost always qualified.

Multiplicative characterEdit

A multiplicative character (or linear character, or simply character) on a group G is a group homomorphism from G to the multiplicative group of a field (Artin 1966), usually the field of complex numbers. If G is any group, then the set Ch(G) of these morphisms forms an abelian group under pointwise multiplication.

This group is referred to as the character group of G. Sometimes only unitary characters are considered (thus the image is in the unit circle); other such homomorphisms are then called quasi-characters. Dirichlet characters can be seen as a special case of this definition.

Multiplicative characters are linearly independent, i.e. if   are different characters on a group G then from   it follows that  .

Character of a representationEdit

The character   of a representation   of a group G on a finite-dimensional vector space V over a field F is the trace of the representation   (Serre 1977), i.e.

  for  

In general, the trace is not a group homomorphism, nor does the set of traces form a group[citation needed]. The characters of one-dimensional representations are identical to one-dimensional representations, so the above notion of multiplicative character can be seen as a special case of higher-dimensional characters. The study of representations using characters is called "character theory" and one dimensional characters are also called "linear characters" within this context.

Alternative definitionEdit

If restricted to Finite Abelian Group with   representation in   (i.e.  ), the following alternative definition would be equivalent to the above (For Abelian groups, every matrix representation decomposes into a direct sum of   representations. For non-Abelian group, the original definition would be more general than this one):

A character   of Group   is a mapping   such that   for all  

If   is a finite Abelian group, the characters play the role of harmonics. For infinite Abelian Group, the above would be replaced by   where   is the Circle group.

See alsoEdit

ReferencesEdit

  1. ^ "character in nLab". ncatlab.org. Retrieved 2017-10-31.

External linksEdit