# Contents

## Definition

A projective representation of a group $G$ is a representation up to a central term: a group homomorphism $G\to PGL(V)$, where $V$ is a $\mathbb{K}$-vector space. Via the projection $GL(V)\to PGL(V)=GL(V)/\mathbb{K}^*$, every linear representation of $G$ induces a projective representation.

By the fibration sequence

$\array{ \mathbf{B}\mathbb{K}^*&\to & \mathbf{B}GL(V)\\ \downarrow&& \downarrow\\ *&\to&\mathbf{B}PGL(V) }$

the obstruction to lift a projective representation of $G$ to a linear representation is represented by an element $c_\rho$ in $H^2(G,\mathbb{K}^*)$.

A refinement of this idea consists in looking at the 2-groupoid $\mathbf{B}GL(V)//\mathbf{K}^*$. Then a functor $(\rho,\lambda):\mathbb{B}G\to \mathbf{B}GL(V)//\mathbf{K}^*$ consists of two maps $\rho:G\to GL(V)$ and $\lambda:G\times G\to \mathbb{K}^*$ such that $\rho(g)\rho(h)=\lambda(g,h)\rho(g h)$, and $\lambda$ is a 2-cocycle on $G$ with values in $\mathbb{K}^*$ representing the cohomology class $c_\rho$.

Revised on July 17, 2013 21:38:29 by Urs Schreiber (82.169.65.155)