A Faster Tree-Decomposition Based Algorithm for Counting Linear Extensions

We investigate the problem of computing the number of linear extensions of a givenn-element poset whose cover graph has treewidtht. We present an algorithm that runs in time$${\tilde{O}}(n^{t+3})$$O~(nt+3)for any constantt; the notation$${\tilde{O}}$$O~hides polylogarithmic factors. Our algorithm applies dynamic programming along a tree decomposition of the cover graph; the join nodes of the tree decomposition are handled by fast multiplication of multivariate polynomials. We also investigate the algorithm from a practical point of view. We observe that the running time is not well characterized by the parametersnandtalone: fixing these parameters leaves large variance in running times due to uncontrolled features of the selected optimal-width tree decomposition. We compare two approaches to select an efficient tree decomposition: one is to include additional features of the tree decomposition to build a more accurate, heuristic cost function; the other approach is to fit a statistical regression model to collected running time data. Both approaches are shown to yield a tree decomposition that typically is significantly more efficient than a random optimal-width tree decomposition.