Hello! Has anyone already looked into this thing?
The bug appears to slow down all matrix computations related to pariGP. For example it gets stuck computing the eigenvalues of a matrix as well. It's quite annoying. The pariGP guys confirmed it's a bug in pari so we should update the pari package that sage ships! Best, Jernej On Thursday, 6 September 2012 14:12:07 UTC+2, Jernej Azarija wrote: > > Consider the following program > > sage: G = graphs.OddGraph(4) > sage: G.spanning_trees_count() > 336140000000000000 > > It takes approximately 8 minutes to compute the number of spanning trees > using this method. However, the number of spanning trees can be computed > directly from the charpoly of the respective Kirchhoff matrix > > sage: (G.kirchhoff_matrix()).charpoly().coeffs()[1]/35 > 336140000000000000 > > which takes an instant (less than a second) to compute. > > Looking at the way spanning_trees_count is implemented within > generic_graph.py, one can see: > > M=self.kirchhoff_matrix() > M.subdivide(1,1) > M2 = M.subdivision(1,1) > return abs(M2.determinant()) # <-The abs() here is redundant someone > should remove it > > To speed the computation one can pass the parameter algorithm='padic' to > the determinant() method and get the result of spanning_trees_count() > instantly. > > That said, I am wondering if this is perhaps a bug in the default > implementation of determinant()? It seems strange to me that it takes 8 > minutes to compute a determinant of a 34x34 matrix while other algorithms > do it within a second. > > -- You received this message because you are subscribed to the Google Groups "sage-devel" group. To post to this group, send email to sage-devel@googlegroups.com. To unsubscribe from this group, send email to sage-devel+unsubscr...@googlegroups.com. Visit this group at http://groups.google.com/group/sage-devel?hl=en.
