An O(n^2) Time Algorithm for Alternating B\"uchi Games
  Computing the winning set for B{\"u}chi objectives in alternating games on graphs is a central problem in computer aided verification with a large number of applications. The long standing best known upper bound for solving the problem is $\tilde{O}(n \cdot m)$, where $n$ is the number of vertices and $m$ is the number of edges in the graph. We are the first to break the $\tilde{O}(n\cdot m)$ bound by presenting a new technique that reduces the running time to $O(n^2)$. This bound also leads to an $O(n^2)$ algorithm time for computing the set of almost-sure winning vertices in alternating games with probabilistic transitions (improving an earlier bound of $\tilde{O}(n\cdot m)$) and in concurrent graph games with constant actions (improving an earlier bound of $O(n^3)$). We also show that the same technique can be used to compute the maximal end-component decomposition of a graph in time $O(n^2)$. Finally, we show how to maintain the winning set for B{\"u}chi objectives in alternating games under a sequence of edge insertions or a sequence of edge deletions in O(n) amortized time per operation. This is the first dynamic algorithm for this problem. 
Krishnendu Chatterjee and Monika Henzinger
cs.GT cs.DS cs.FL