Counting Closed Trails

In English A closed trail is a connected graph whose every vertex is incident to an even number of edges. We give a deterministic algorithm that in time $2^{m/2}poly(m,n)$ finds the number of closed trails in a given graph G with n vertices and m edges. Moreover, within the same time bound we can determine every possible vertex set of a closed trail in G, together with the associated number of closed trails. Our algorithm can be used to deterministically find the longest cycle in an n-vertex claw-free graph in time $2^{n/2}poly(m,n)$ via a framework presented by Broersma et al. (in press, http://dx.doi.org/10.1007/s00453-011-9576-4) 5, thus improving both upon the $O(1.66^n)$ time randomized algorithm for general graphs (Björklund, 2010, http://dx.doi.org/10.1109/FOCS.2010.24, 1), as well as the $O(1.69^n)$ time deterministic algorithm for claw-free graphs by Broersma et al.