GSoC with Flint – Week 10
This week I worked more on the Smith normal form algorithm I talked about last week. I implemented Iliopoulos’ algorithm for computation of the Smith form modulo an arbitrary integer, this procedure is used in a couple of places as part of Saunders and Wan’s “engineered” algorithm. Firstly we use a prime power modulus to find the Smith normal form locally for small primes (i.e. those less than 100), the modular approach is also used for the rough part (concerning all the other primes) when the largest non-zero invariant factor is small compared to the size of the matrix. This algorithm is now working correctly, though the question of how to test it properly given its Monte Carlo nature is one that will require some thought. Currently whenever I have encountered a matrix for which the output of Saunders and Wan doesn’t match the deterministic algorithms’ outputs it has turned out to be a code bug rather than a side effect of the algorithm’s randomness. I suppose allowing for a reasonable number of failures beyond the expected number in test code would be one approach, but of course there will still be occasions when the number of failures exceeds this and allowing a large number of extra failures could allow real bugs to creep in.
For the next couple of days I’m going to work a little more on Smith forms, hopefully implementing Storjohann’s banded method for finding Smith forms of upper triangular matrices, this could be coupled with a HNF algorithm to give another (deterministic!) algorithm for the Smith form of a general matrix. I also need to clean up the Saunders and Wan code a bit as there are still a number of inefficiencies. I have not got a valence method included in this algorithm as this would require implementing a few other things (such as minimal polynomials), but the option is certainly there and it would easily slot in to the current code.