Uniformity and functional equations for local zeta functions of $\mathfrak {K}$-split algebraic groups

We study the local zeta functions of an algebraic group $\mathcal {G}$ defined over $\mathfrak {K}$ together with a faithful $\mathfrak {K}$-rational representation $\rho$ for a finite extension $\mathfrak {K}$ of ${\Bbb Q}$. These are given by integrals over $\mathfrak {p}$-adic points of ${\cal G}$ determined by $\rho$ for a prime $\mathfrak {p}$ of $\mathfrak {K}$. We prove that the local zeta functions are almost uniform for all $\mathfrak {K}$-split groups whose unipotent radical satisfies a certain lifting property. This property is automatically satisfied if $\mathcal {G}$ is reductive. We provide a further criterion in terms of invariants of $\mathcal {G}$ and $\rho$ which guarantees that the local zeta functions satisfy functional equations for almost all primes of $\mathfrak {K}$. We obtain these results by using a $\mathfrak {p}$-adic Bruhat decomposition of Iwahori and Matsumoto to express the zeta function as a weighted sum over the Weyl group $W$ associated to $\mathcal {G}$ of generating functions over lattice points of a polyhedral cone. The functional equation reflects an interplay between symmetries of the Weyl group and reciprocities of the combinatorial object. We construct families of groups with representations violating our second structural criterion whose local zeta functions do not satisfy functional equations. Our work generalizes results of Igusa and du Sautoy and Lubotzky and has implications for zeta functions of finitely generated torsion-free nilpotent groups.