Abstract
Herz–Schur multipliers of a locally compact group have a well developed theory coming from a large literature; they have proved very useful in the study of the reduced C∗algebra of a locally compact group. There is also a rich connection to Schur multipliers,which have been studied since the early twentieth century, and have a large number of applications.We develop a theory of Herz–Schur multipliers of a C∗dynamical system, extending the classical Herz–Schur multipliers, making Herz–Schur multiplier techniques available to study a much larger class of C∗algebras. Furthermore, we will also introduce and study generalised Schur multipliers, and derive links between these two notions which extend the classical results describing Herz–Schur multipliers in terms of Schur multipliers.This theory will be developed in as much generality as possible, with reference to the classical motivation.
After introducing all the necessary concepts we begin the investigation by defining generalised Schur multipliers. The main result is a dilation type characterisation of these multipliers; we also show how such multipliers can be represented using HilbertC∗modules. Next we introduce and study generalised Herz–Schur multipliers, first extending a classical result involving the representation theory of SU(2), before studying how such functions are related to our generalised Schur multipliers. We give a characterisation of generalised Herz–Schur multipliers as a certain class of the generalised Schur multipliers, and obtain a description of precisely which Schur multipliers belong to this class. Finally, we consider some ways in which the generalised multipliers can arise; firstly, from the classical multipliers which provide our motivation, secondly, from the Haagerup tensor product of a C∗algebra with itself, and finally from positivity considerations.We show that our theory behaves well with respect to positivity and give conditions under which our multipliers are automatically positive in a natural sense
Date of Award  2017 

Original language  English 
Awarding Institution 

Supervisor  Ivan Todorov (Supervisor) & Thomas Huettemann (Supervisor) 
Keywords
 Schur
 multiplier
 HerzSchur
 dynamical system