Faithful representation - Alchetron, the free social encyclopedia

In mathematics, especially in an area of abstract algebra known as representation theory, a faithful representation ρ of a group G on a vector space V is a linear representation in which different elements g of G are represented by distinct linear mappings ρ(g).

In more abstract language, this means that the group homomorphism

ρ: G → GL(V)

is injective (or one-to-one).

Caveat: While representations of G over a field K are de facto the same as K [ G ] -modules (with K [ G ] denoting the group algebra of the group G), a faithful representation of G is not necessarily a faithful module for the group algebra. In fact each faithful K [ G ] -module is a faithful representation of G, but the converse does not hold. Consider for example the natural representation of the symmetric group S_n in n dimensions by permutation matrices, which is certainly faithful. Here the order of the group is n! while the n×n matrices form a vector space of dimension n². As soon as n is at least 4, dimension counting means that some linear dependence must occur between permutation matrices (since 24 > 16); this relation means that the module for the group algebra is not faithful.

Properties

A representation V of a finite group G over an algebraically closed field K of characteristic zero is faithful (as a representation) if and only if every irreducible representation of G occurs as a subrepresentation of SⁿV (the n-th symmetric power of the representation V) for a sufficiently high n. Also, V is faithful (as a representation) if and only if every irreducible representation of G occurs as a subrepresentation of

V ⊗ n = V ⊗ V ⊗ ⋯ ⊗ V ⏟ n times

(the n-th tensor power of the representation V) for a sufficiently high n.

References

Faithful representation Wikipedia

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