Edge-isoperimetric problem for Cayley graphs and generalized Takagi function

Let $G$ be a finite abelian abelian group of exponent $m\ge 2$. For subsets $A,S\subset G$, denote by $\partial_S(A)$ the number of edges from $A$ to its complement $G\setminus A$ in the directed Cayley graph, induced by $S$ on $G$. We show that if $S$ generates $G$, and $A$ is non-empty, then $\partial_S(A) \ge \frac{e}m\,|A|\ln\frac{|G|}{|A|}.$ Here the coefficient $e=2.718...$ is best possible and cannot be replaced with a number larger than $e$. For homocyclic groups $G$ of exponent $m$, we find an explicit closed-form expression for $\partial_S(A)$ in the case where $S$ is a "standard" generating subset of $G$, and $A$ is an initial segment of $G$ with respect to the lexicographic order, induced by $S$ on $G$. Namely, we show that in this situation $\partial_S(A) = |G|\,ω_m(|A|/|G|),$ where $ω_2$ is the Takagi function, and $ω_m$ for $m\ge 3$ is an appropriate generalization thereof. This particular case is of special interest, since for $m\in\{2,3,4\}$ it is known to yield the smallest possible value of $\partial_S(A)$, over all sets $A\subset G$ of given size. We give this classical result a new proof, somewhat different from the standard one. We also give a new, short proof of the Boros-Pales inequality $ω_2(\frac{x+y}2) \le \frac{ω_2(x) + ω_2(y)}2 + \frac12\,|y-x|,$ establish an extremal characterization of the Takagi function as the (pointwise) maximal function, satisfying this inequality and the boundary condition $\max\{ω_2(0),ω_2(1)\}\le 0$, and obtain similar results for the 3-adic analog $ω_3$ of the Takagi function.

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