Type I neighbors of extremal type II codes of length 40 derived from Hadamard matrices

Self-dual codes C1,C2 of length n are called neighbors to each other if C1∩C2 has dimension (n/2−1). With the aide of a computer, we search for Type I neighbors of some extremal Type II codes of length 40 which are derived from Hadamard matrices of order 20. As a result, we get extremal Type I neighbors (up to equivalence) that have weight enumerators W40(y)=1+(125+16β)y8+⋯ with β=0,…,4,6,8,10.a