A universal theory of switching for combinatorial objects, and applications to complex Hadamard matrices

TL;DR

This paper introduces a universal switching framework applicable to various combinatorial objects, extending existing methods to complex Hadamard matrices and addressing open problems in their structure.

Contribution

It develops a unified switching theory for combinatorial objects, generalizes Orrick's techniques to complex Hadamard matrices, and explores trade sizes in these matrices.

Findings

Extended Orrick's switching methods to complex Hadamard matrices.

Constructed new inequivalent complex Hadamard matrices using the universal switching.

Addressed an open problem on the permissible size of trades in complex Hadamard matrices.

Abstract

The concept of switching has arisen in several different areas within combinatorics. The act of switching usually transforms a combinatorial object into a non-isomorphic object of the same type, in a way that some key property is preserved. Godsil-McKay switching of graphs preserves the spectrum, switching of designs preserves their parameters, and switching of binary codes preserves the minimum distance. For Hadamard matrices, the switching techniques introduced by Orrick proved to be an incredibly powerful tool in generating inequivalent Hadamard matrices. In this paper, we introduce a universal definition of switching that can be adapted to incorporate these known types of switching. Through this language, we extend Orrick's methods to Butson Hadamard and complex Hadamard matrices. We introduce switchings of these matrices that can be used to construct new, inequivalent matrices. We also consider the concept of trades in complex Hadamard matrices in this terminology, and address an open problem on the permissible size of a trade.