Sharp Poincare-Wirtinger inequalities on complete graphs

Abstract

Let $K_n=(V,E)$ be the complete graph with $n\geq 3$ vertices (here $V$ and $E$ denote the set of vertices and edges of $K_n$ respectively). We find the optimal value ${\bf{C}}_{n,p}$ such that the inequality $$\|f-m_f\|_p\le {\bf C}_{n,p}{\rm Var}_{p}f$$ holds for every $f:V\to \mathbb{R},$ where ${\rm Var}_p$ stands for the $p$-variation, and $m_f$ stands for the average value of $f$, for all $p\in[1,3+\delta^1_n)\cup (3+\delta^2_n,+\infty)$, for $\delta^1_n=\frac{1}{2n^2\log(n)}+O(1/n^3)$ and $\delta^2_n=\frac{2}{n}+O(1/n^2).$ Moreover, we characterize all the maximizer functions in that case. The behavior of the maximizers is different in each of the intervals $(1,2)$, $(2,3+\delta^{1}_n)$ and $(3+\delta^{2}_n,\infty).$