Analytic properties of zeta functions and subgroup growth
  In this paper we introduce some new methods to understand the analytic behaviour of the zeta function of a group. We can then combine this knowledge with suitable Tauberian theorems to deduce results about the growth of subgroups in a nilpotent group. In order to state our results we introduce the following notation. For \alpha a real number and N a nonnegative integer, define   s_N^\alpha(G) = sum_{n=1}^N a_n(G)/n^\alpha.   Main Theorem: Let G be a finitely generated nilpotent infinite group.   (1) The abscissa of convergence \alpha(G) of \zeta_G(s) is a rational number and \zeta_G(s) can be meromorphically continued to Re(s)>\alpha(G)-\delta for some \delta >0. The continued function is holomorphic on the line \Re(s) = (\alpha)G except for a pole at s=\alpha(G).   (2) There exist a nonnegative integer b(G) and some real numbers c,c' such that   s_{N}(G) ~ c N^{\alpha(G)}(\log N)^{b(G)}   s_{N}^{\alpha(G)}(G) ~ c' (\log N)^{b(G)+1}   for N\rightarrow \infty . 
Marcus du Sautoy, Fritz Grunewald