I am reading The Fourier Transform and Equations over Finite Abelian Groups which aims to prove Fermat's Last Theorem on finite fields using harmonic analysis. A central notion is this $\phi(A)$ defined for a set $A$ like this:
Let $G$ be a finite abelian group, $\hat G$ its dual. For $f\in\mathbb{C}^{G}$ we note $\hat f\in\mathbb{C}^{\hat G}$ its Fourier transform. And for a set $A\subset G$ we note $1_{A}$ its indicator.
Then $\phi(A)$=$\max\bigg\lbrace|\hat 1_A(\chi)|\bigg | \chi\in\hat G, \chi\neq1\bigg\rbrace$
This function is used to find interesting bounds on the number of solution of equations over abelian groups.
I would like to learn more about it but I can't find how this thing is named.
Where can I learn more about it?