# Brauer–Nesbitt theorem

Brauer–Nesbitt theorem In mathematics, the Brauer–Nesbitt theorem can refer to several different theorems proved by Richard Brauer and Cecil J. Nesbitt in the representation theory of finite groups.

In modular representation theory, the Brauer–Nesbitt theorem on blocks of defect zero states that a character whose order is divisible by the highest power of a prime p dividing the order of a finite group remains irreducible when reduced mod p and vanishes on all elements whose order is divisible by p. Moreover, it belongs to a block of defect zero. A block of defect zero contains only one ordinary character and only one modular character.

Another version states that if k is a field of characteristic zero, A is a k-algebra, V, W are semisimple A-modules which are finite dimensional over k, and TrV = TrW as elements of Homk(A,k), then V and W are isomorphic as A-modules.

Let {displaystyle G} be a group and {displaystyle E} be some field. If {displaystyle rho _{i}:Gto GL_{n}(E),i=1,2} are two finite-dimensional semisimple representations such that the characteristic polynomials of {displaystyle rho _{1}(g)} and {displaystyle rho _{2}(g)} coincide for all {displaystyle gin G} , then {displaystyle rho _{1}} and {displaystyle rho _{2}} are isomorphic representations. If {displaystyle char(E)=0} or {displaystyle char(E)>n} , then the condition on the characteristic polynomials can be changed to the condition that the traces of {displaystyle rho _{1}(g)} and {displaystyle rho _{2}(g)} coincide for all {displaystyle gin G} . As a consequence, let {displaystyle rho :Gal(K^{s}/K)to GL_{n}({overline {mathbb {Q} }}_{l})} be a semisimple (continuous) {displaystyle l} -adic representations of the absolute Galois group of some field {displaystyle K} , unramified outside some finite set of primes {displaystyle Ssubset M_{K}} . Then the representation is uniquely determined by the values of the traces of {displaystyle rho (Frob_{p})} for {displaystyle pin M_{K}^{0}-S} (also using the Chebotarev density theorem).

References Curtis, Reiner, Representation theory of finite groups and associative algebras, Wiley 1962. Brauer, R.; Nesbitt, C. On the modular characters of groups. Ann. of Math. (2) 42, (1941). 556-590. This abstract algebra-related article is a stub. You can help Wikipedia by expanding it.

Categories: Representation theory of finite groupsTheorems about algebrasTheorems in group theoryAbstract algebra stubs

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