Let Alt k be the alternating group of degree k. In this paper we prove that the order complex of the coset poset of Alt k is non-contractible for a big family of k P N, including the numbers of the form k p m where m P f3; . . . ; 35g and p > k=2. In order to prove this result, we show that P G ( À 1) does not vanish, where P G (s) is the Dirichlet polynomial associated to the group G. Moreover, we extend the result to some monolithic primitive groups whose socle is a direct product of alternating groups.MATHEMATICS SUBJECT CLASSIFICATION (2010). 20D30, 20P05, 11M41. defined inductively by m G (G) 1, m G (H) À P K>H m G (K). The counterpart of the Dirichlet polynomial is called the probabilistic zeta function of G (see [1] and [6]).In particular, Brown ([2], § 3) showed that P G ( À 1) Àx(g(G)):It is a well-known fact that if D(g(G)) is contractible, then its reduced Euler characteristicx(g(G)) is zero. Hence, if P G ( À 1) T 0, then the simplicial complex associated to the group G is non-contractible. Moreover, in [2], Brown conjectured the following. CONJECTURE 1. If G is a finite group, then P G ( À 1) T 0. Hence the order complex of the coset poset of G is non-contractible.On the (non-)Contractibility of the Order Complex etc.