English

On Projective representations of direct product of groups

Representation Theory 2023-11-21 v1

Abstract

Let G=G1×G2G=G_1 \times G_2 be a finite group. We know that the second cohomology group H2(G,C×)H^2(G,\mathbb C^\times) is isomorphic to H2(G1,C×)×H2(G2,C×)×Hom(G1/G1ZG2/G2,C×).H^2(G_1,\mathbb C^\times) \times H^2(G_2,\mathbb C^\times) \times Hom(G_1/G_1' \otimes_\mathbb Z G_2/G_2', \mathbb C^\times ). A 22-cocycle α\alpha of GG is called a bilinear cocycle if the corresponding cohomology class [α][\alpha] of H2(G,C×)H^2(G,\mathbb C^\times) lies in Hom(G1/G1ZG2/G2,C×)Hom(G_1/G_1' \otimes_\mathbb Z G_2/G_2', \mathbb C^\times). In this article, our aim is to construct an irreducible complex projective representation ρ\rho of GG for bilinear cocycles α\alpha. If G1G_1 is any abelian pp-group and G2G_2 is an elementary abelian pp-group, then we give a construction of ρ\rho for bilinear cocycles α\alpha of GG. For a subgroup HH of GG of index p2\leq p^2, we also count the number of cohomology classes [α][\alpha] for which the irreducible projective representations behave the same while restricting on HH. Finally, we consider any pp-group G=G1×G2G=G_1\times G_2, and we discuss how the above construction helps us to describe an irreducible α\alpha-representation of GG when [α][\alpha] is of order pp or G2/G2G_2/G_2' is elementary abelian. We also discuss several examples as an application of the above results.

Keywords

Cite

@article{arxiv.2311.11049,
  title  = {On Projective representations of direct product of groups},
  author = {Sumana Hatui},
  journal= {arXiv preprint arXiv:2311.11049},
  year   = {2023}
}

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16 pages