Rational and quasi-permutation representations of holomorphs of cyclic $p$-groups

Document Type : Research Paper


1 School of Mathematics, Harish-Chandra Research Institute, HBNI, Chhatnag Road, Jhunsi, Allahabad, 211019, India

2 Stat-Math Unit, Indian Statistical Institute, Bangalore Centre, 8-th Mile Mysore Road , Bangalore, 560059, India


‎For a finite group $G$‎, ‎three of the positive integers governing its‎ ‎representation theory over $\mathbb{C}$ and over $\mathbb{Q}$ are‎ ‎$p(G),q(G),c(G)$‎. ‎Here‎, ‎$p(G)$ denotes the {\it minimal degree} of a‎ ‎faithful permutation representation of $G$‎. ‎Also‎, ‎$c(G)$ and $q(G)$‎ ‎are‎, ‎respectively‎, ‎the minimal degrees of a faithful representation‎ ‎of $G$ by quasi-permutation matrices over the fields $\mathbb{C}$‎ ‎and $\mathbb{Q}$‎. ‎We have $c(G)\leq q(G)\leq p(G)$ and‎, ‎in general‎, ‎either inequality may be strict‎. ‎In this paper‎, ‎we study the‎ ‎representation theory of the group $G =$ Hol$(C_{p^{n}})$‎, ‎which is‎ ‎the holomorph of a cyclic group of order $p^n$‎, ‎$p$ a prime‎. ‎This group is metacyclic when $p$ is odd and metabelian but not‎ ‎metacyclic when $p=2$ and $n \geq 3$‎. ‎We explicitly describe the set‎ ‎of all isomorphism types of irreducible representations of $G$‎ ‎over the field of complex numbers $\mathbb{C}$ as well as the‎ ‎isomorphism types over the field of rational numbers $\mathbb{Q}$‎. ‎We compute the Wedderburn decomposition of the rational group‎ ‎algebra of $G$‎. ‎Using the descriptions of the irreducible‎ ‎representations of $G$ over $\mathbb{C}$ and over $\mathbb{Q}$‎, ‎we‎ ‎show that $c(G) = q(G) = p(G) = p^n$ for any prime $p$‎. ‎The proofs‎ ‎are often different for the case of $p$ odd and $p=2$‎.


Main Subjects

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