The invariants of the Clifford groups

The automorphism group of the Barnes-Wall lattice L-m in dimension 2(m)(m n ot equal 3 ) is a subgroup of index 2 in a certain "Clifford group" C-m of structure 2(+)(1+2m) . O+(2m,2). This group and its complex analogue X-m of structure (2(+)(1+2m)YZ(8)) . Sp(2m, 2) have arisen in recent years in con nection with the construction of orthogonal spreads, Kerdock sets, packings in Grassmannian spaces, quantum codes, Siegel modular forms and spherical designs. In this paper we give a simpler proof of Runge's 1996 result that the space of invariants for C-m of degree 2k is spanned by the complete wei ght enumerators of the codes C x F-2m, where C ranges over all binary self- dual codes of length 2k; these are a basis if m greater than or equal to k- 1. We also give new constructions for L-m and C-m: let M be the Z[root2]-la ttice with Gram matrix [GRAPHICS] Then L-m is the rational part of M-xm, and C-m = Aut (M-xm ). Also, if C is a binary self-dual code not generated by vectors of weight 2, then C-m is precisely the automorphism group of the complete weight enumerator of C x F -2(m). There are analogues of all these results for the complex group X-m, with "doubly-even self-dual code" instead of "self-dual code."