# Homomorphisms of matrix algebras and constructions of Butson-Hadamard matrices

## TL;DR

This paper simplifies and generalizes a method for constructing larger Butson-Hadamard matrices from smaller ones, removing previous restrictions on the prime factorization of the matrix entries' roots of unity.

## Contribution

It provides a more general proof for constructing larger Butson-Hadamard matrices, extending previous results to cases with multiple prime divisors without restrictions.

## Key findings

- Simplified proof of a matrix construction method
- Extended the construction to matrices with multiple prime divisors
- Removed previous restrictions on prime factorization

## Abstract

An $n \times n$ matrix $H$ is Butson-Hadamard if its entries are $k^{\text{th}}$ roots of unity and it satisfies $HH^* = nI_n$. Write $BH(n, k)$ for the set of such matrices. Suppose that $k = p^{\alpha}q^{\beta}$ where $p$ and $q$ are primes and $\alpha \geq 1$. A recent result of {\"O}sterg{\aa}rd and Paavola uses a matrix $H \in BH(n,pk)$ to construct $H' \in BH(pn, k)$. We simplify the proof of this result and remove the restriction on the number of prime divisors of $k$. More precisely, we prove that if $k = mt$, and each prime divisor of $k$ divides $t$, then we can construct a matrix $H' \in BH(mn, t)$ from any $H \in BH(n,k)$.