Paper 3, Section II, G

Representation Theory
Part II, 2013

Suppose that (ρ1,V1)\left(\rho_{1}, V_{1}\right) and (ρ2,V2)\left(\rho_{2}, V_{2}\right) are complex representations of the finite groups G1G_{1} and G2G_{2} respectively. Use ρ1\rho_{1} and ρ2\rho_{2} to construct a representation ρ1ρ2\rho_{1} \otimes \rho_{2} of G1×G2G_{1} \times G_{2} on V1V2V_{1} \otimes V_{2} and show that its character satisfies

χρ1ρ2(g1,g2)=χρ1(g1)χρ2(g2)\chi \rho_{1} \otimes \rho_{2}\left(g_{1}, g_{2}\right)=\chi_{\rho_{1}}\left(g_{1}\right) \chi \rho_{2}\left(g_{2}\right)

for each g1G1,g2G2g_{1} \in G_{1}, g_{2} \in G_{2}.

Prove that if ρ1\rho_{1} and ρ2\rho_{2} are irreducible then ρ1ρ2\rho_{1} \otimes \rho_{2} is irreducible as a representation of G1×G2G_{1} \times G_{2}. Moreover, show that every irreducible complex representation of G1×G2G_{1} \times G_{2} arises in this way.

Is it true that every complex representation of G1×G2G_{1} \times G_{2} is of the form ρ1ρ2\rho_{1} \otimes \rho_{2} with ρi\rho_{i} a complex representation of GiG_{i} for i=1,2?i=1,2 ? Justify your answer.