Mappings of Butson-type Hadamard matrices

A BH(q,n) Butson-type Hadamard matrix H is an n×n matrix over the complex qth roots of unity that fulfils HH∗=nIn. It is well known that a BH(4,n) matrix can be used to construct a BH(2,2n) matrix, that is, a real Hadamard matrix. This method is here generalised to construct a BH(q,pn) matrix from a BH(pq,n) matrix, where q has at most two distinct prime divisors, one of them being p. Moreover, an algorithm for finding the domain of the mapping from its codomain in the case p=q=2 is developed and used to classify the BH(4,16) matrices from a classification of the BH(2,32) matrices.