Groups having complete bipartite divisor graphs for their conjugacy class sizes

Given a finite group G, the bipartite divisor graph for its conjugacy class sizes is the bipartite graph with bipartition consisting of the set of conjugacy class sizes of G-Z (where Z denotes the centre of G) and the set of prime numbers that divide these conjugacy class sizes, and with {p,n} being an edge if gcd(p,n)\neq 1. In this paper we construct infinitely many groups whose bipartite divisor graph for their conjugacy class sizes is the complete bipartite graph K_{2,5}, giving a solution to a question of Taeri.Comment: 5 page

On nonsolvable groups whose prime degree graphs have four vertices and one triangle

‎Let $G$ be a finite group‎. ‎The prime degree graph of $G$‎, ‎denoted‎ ‎by $Delta(G)$‎, ‎is an undirected graph whose vertex set is $rho(G)$ and there is an edge‎ ‎between two distinct primes $p$ and $q$ if and only if $pq$ divides some irreducible‎ ‎character degree of $G$‎. ‎In general‎, ‎it seems that the prime graphs‎ ‎contain many edges and thus they should have many triangles‎, ‎so one of the cases that would be interesting is to consider those finite groups whose prime degree graphs have a small number of triangles‎. ‎In this paper we consider the case where for a nonsolvable group $G$‎, ‎$Delta(G)$ is a connected graph which has only one triangle and four vertices‎

Binary linear codes with a fixed point free permutation automorphism of order three

We investigate the structural properties of binary linear codes whose permutation automorphism group has a fixed point free automorphism of order $3$. We prove that up to dimension or codimension $4$, there is no binary linear code whose permutation automorphism group is generated by a fixed point free permutation of order $3$. We also prove that there is no binary $5$-dimensional linear code whose length is at least $30$ and whose permutation automorphism group is generated by a fixed point free permutation of order $3$.Comment: 10 page