On involutions in extremal self-dual codes and the dual distance of semi self-dual codes

Martino Borello; Gabriele Nebe

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arXiv · 1401.6036

On involutions in extremal self-dual codes and the dual distance of semi self-dual codes

Martino Borello·Gabriele Nebe

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Abstract

A classical result of Conway and Pless is that a natural projection of the fixed code of an automorphism of odd prime order of a self-dual binary linear code is self-dual. In this paper we prove that the same holds for involutions under some (quite strong) conditions on the codes. In order to prove it, we introduce a new family of binary codes: the semi self-dual codes. A binary self-orthogonal code is called semi self-dual if it contains the all-ones vector and is of codimension 2 in its dual code. We prove upper bounds on the dual distance of semi self-dual codes. As an application we get the following: let C be an extremal self-dual binary linear code of length 24m and s in Aut(C) be a fixed point free automorphism of order 2. If m is odd or if m=2k with binom{5k-1}{k-1} odd then C is a free F_2 -module. This result has quite strong consequences on the structure of the automorphism group of such codes.

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Martino Borello, Gabriele Nebe. 2014-11-24. On involutions in extremal self-dual codes and the dual distance of semi self-dual codes. https://arxiv.org/abs/1401.6036

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