Nonlinear functions in abelian groups and relative difference sets.
This paper shows that the main results on nonlinear functions on finite fields can be generalized to abelian groups using the discrete Fourier tranform. The paper is a very interesting survey on (generalisations of) relative difference sets and nonlinear functions introducing new points of view. Let \(K\) and \(N\) be abelian (additive) groups with \(| K| =m\), \(| N| =n\) and \(f:K\to N\) be a function. Let \(D_f=\{\langle g,f(g)\rangle \mid g\in K\}\subset G=K\times N\). Call a function \(f\) perfect nonlinear if \(\delta_f(a,b)=| \{g\in K\mid f(g+a)-f(g)=b\}| \) is equal to \(m/n\) for all \(a\in K\setminus\{0\}\) and \(b\in N\). Now \(D_f\) is a splitting \((m,n,m,m/n)\)-DS in \(G\) relative to \(\{0\}\times N\) if and only if \(f\) is perfect nonlinear. This suggest to use the discrete Fourier transform to study more general sets \(D_f\). A key of the paper is the following definition: a function \(f:K\to N\) is an almost perfect nonlinear function if \(\sum_{a,b}[\delta_f(a,b)]^2\leq\sum_{a,b}[\delta_g(a,b)]^2\) for all functions \(g:K\to N\). Too many definitions and notations are necessary to go here into detail; we can only note that using this new point of view many proofs are more transparent and connections with relative difference sets become apparent.
scientific article; zbMATH DE number 2062822
scientific article