Hi sagers, First let me say thanks for the great piece of software that is sage. You all do a tremendous job and maybe don't get thanked enough.
There appears to be a bug in sparse matrix multiplication over small finite fields: sage: p = next_prime(2**15); p 32771 sage: M = Matrix(GF(p), 1,3, lambda i,j: -1, sparse=True); M [32770 32770 32770] sage: M*M.transpose() [32738] # INCORRECT, should be 3 sage: M*M.transpose() + 2**32 [3] # it was off by 2^32, hmmm... Being off by 2^32 hints that there is an integer overflow or signedness issue, and indeed I believe the culprit is in the _matrix_times_matrix_ method on line 387 of matrix_modn_sparse.pyx <https://github.com/sagemath/sage/blob/master/src/sage/matrix/matrix_modn_sparse.pyx#L387>, where a cdef int s is added up without reducing modulo p. I am currently running release 8.0 on a 64-bit Debian machine. I had some old versions of sage lying around and the bug seems to be present in versions going back at least to 5.6; it's not a recent change. Thanks for taking a look! -Dan -- You received this message because you are subscribed to the Google Groups "sage-devel" group. To unsubscribe from this group and stop receiving emails from it, send an email to sage-devel+unsubscr...@googlegroups.com. To post to this group, send email to sage-devel@googlegroups.com. Visit this group at https://groups.google.com/group/sage-devel. For more options, visit https://groups.google.com/d/optout.
