Polynomiality of the faithful dimension of nilpotent groups over finite truncated valuation rings

Abstract: The faithful dimension of a finite group $\mathrm G$ over $\mathbb C$, denoted by $m_\mathrm{faithful}(\mathrm G)$, is the smallest integer $n$ such that $\mathrm G$ can be embedded in $\mathrm{GL}n(\mathbb C)$. Continuing our previous work (arXiv:1712.02019), we address the problem of determining the faithful dimension of a finite $p$-group of the form $\mathcal G_R:=\exp(\mathfrak g_R)$ associated to $\mathfrak g_R:=\mathfrak g \otimes\mathbb Z R $ in the Lazard correspondence, where $\mathfrak g$ is a nilpotent $\mathbb Z$-Lie algebra and $R$ ranges over finite truncated valuation rings. Our first main result is that if $R$ is a finite field with $p^f$ elements and $p$ is sufficiently large, then $m_\mathrm{faithful}(\mathcal G_R)=fg(p^f)$ where $g(T)$ belongs to a finite list of polynomials $g_1,\ldots,g_k$, with non-negative integer coefficients. The list of polynomials is uniquely determined by the Lie algebra $\mathfrak g$. Furthermore, for $1\leq i\leq k$ the set of pairs $(p,f)$ for which $g=g_i$ is a finite union of Cartesian products $\mathcal P\times \mathcal F$, where $\mathcal P$ is a Frobenius set of prime numbers and $\mathcal F$ is a subset of $\mathbb N$ that belongs to the Boolean algebra generated by arithmetic progressions. Next we formulate a conjectural polynomiality property for $m_\mathrm{faithful}(\mathcal G_R)$ in the more general setting where $R$ is a finite truncated valuation ring, and prove special cases of this conjecture. In particular, we show that for a vast class of Lie algebras $\mathfrak g $ that are defined by partial orders, $m_\mathrm{faithful}(\mathcal G_R)$ is given by a single polynomial-type formula. Finally, we compute $m_\mathrm{faithful}(\mathcal G_R)$ precisely in the case where $\mathfrak g$ is the free metabelian nilpotent Lie algebra of class $c$ on $n$ generators and $R$ is a finite truncated valuation ring.