On the classification of binary completely transitive codes with almost-simple top-group.

A code C in the Hamming metric, that is, is a subset of the vertex set V Γ of the Hamming graph Γ = H (m , q) , gives rise to a natural distance partition { C , C 1 , ... , C ρ } , where ρ is the covering radius of C. Such a code C is called completely transitive if the automorphism group Aut (C) acts transitively on each of the sets C , C 1 , ..., C ρ. A code C is called 2-neighbour-transitive if ρ ⩾ 2 and Aut (C) acts transitively on each of C , C 1 and C 2. Let C be a completely transitive code in a binary (q = 2) Hamming graph having full automorphism group Aut (C) and minimum distance δ ⩾ 5. Then it is known that Aut (C) induces a 2-homogeneous action on the coordinates of the vertices of the Hamming graph. The main result of this paper classifies those C for which this induced 2-homogeneous action is not an affine, linear or symplectic group. We find that there are 13 such codes, 4 of which are non-linear codes. Though most of the codes are well-known, we obtain several new results. First, a non-linear completely transitive code that does not explicitly appear in the existing literature is constructed, as well as a related non-linear code that is 2-neighbour-transitive but not completely transitive. Moreover, new proofs of the complete transitivity of several codes are given. Additionally, we consider the question of the existence of distance-regular graphs related to the completely transitive codes appearing in our main result. [ABSTRACT FROM AUTHOR]