Does every quasisimple finite group have a faithful complex irrep

finite-groupsgroup-theoryrepresentation-theorysimple-groups

Every simple finite group has a faithful complex irrep. Indeed any nontrivial irrep of a simple finite group is faithful. That leads me to ask:

Does every quasisimple group have a faithful complex irrep?

Note: I am aware that not all finite groups have a faithful complex irrep. For example, any noncyclic abelian group (e.g. $ C_2 \times C_2 $) has no faithful complex irreps.

Best Answer

No. A quasisimple group has a faithful irreducible representation if and only if the centre is cyclic. Since there are simple groups with non-cyclic Schur multiplier (e.g., $PSL_3(4)$, which has Schur multiplier $4\times 4\times 3$), there are no faithful irreducible representations of $4^2.PSL_3(4)$.

Edit: I should say, morally the answer is 'yes', because finite simple groups have cyclic Schur multiplier. The problem is for some small groups, for example $PSL_3(4)$, $Sz(8)$, $PSU_4(3)$, this isn't true, and also, as one comment just reminded me, the orthogonal groups $\Omega_{4n}^+(q)$ for $q$ odd and $n\geq 2$. But there are only finitely many such examples, apart from the orthogonal groups.

Edit 2: Just to clarify, the situation for quasisimple groups is that they have faithful, irreducible representations if and only if they have a cyclic centre. This is not difficult to see by inducing a faithful linear character for the centre. In general, this is not sufficient. This MathOverflow post gives information about the general case.