Geometry applications of irreducible representations of Lie Groups

In this note we give proofs of the following three algebraic facts which have applications in the theory of holonomy groups and homogeneous spaces: Any irreducibly acting connected subgroup G \subset Gl(n,\rr) is closed. Moreover, if $G$ admits an invariant bilinear form of Lorentzian signature, $G$ is maximal, i.e. it is conjugated to $SO(1,n-1)_0$. We calculate the vector space of $G$-invariant symmetric bilinear forms, show that it is at most $3$-dimensional, and determine the maximal stabilizers for each dimension. Finally, we give some applications and present some open problem

Subgroup growth of lattices in semisimple Lie groups

We give very precise bounds for the congruence subgroup growth of arithmetic groups. This allows us to determine the subgroup growth of irreducible lattices of semisimple Lie groups. In the most general case our results depend on the Generalized Riemann Hypothesis for number fields but we can state the following unconditional theorem: Let $G$ be a simple Lie group of real rank at least 2, different than D_4(\bbc), and let $\Gamma$ be any non-uniform lattice of $G$. Let $s_n(\Gamma)$ denote the number of subgroups of index at most $n$ in $\Gamma$. Then the limit $\lim\limits_{n\to \infty} \frac{\log s_n(\Gamma)}{(\log n)^2/ \log \log n}$ exists and equals a constant $\gamma(G)$ which depends only on the Lie type of $G$ and can be easily computed from its root system.Comment: 34 page