Monday, 30 June 2014

Week of 6/23 Update

This week I implemented fast polynomial interpolation and evaluation. As I mentioned last week the plan is to compute $det(A)$ for a polynomial matrix $A$ by evaluating $det(A(p))$ at a bunch of points $p$ and interpolating the polynomial that runs through those values. If the largest polynomial in $A$ has degree $d$ then we need $d+1$ points. The naive algorithm for evaluating a polynomial takes $O(d)$ time per point for a total of $O(d^{2})$ field operations. Since the polynomials from block-wiedemann have degree of order $\frac{n}{b}$ (here $n$ is the matrix dimension and very big, $b$ is the blocking factor and fairly small) we definitely needed a faster method. Although we can't do an FFT per se since our fields lack points with the requisite properties, there is a similar algorithm (Modern Computer Algebra, algorithms 10.5 and 10.11) that works for any choice of points. The new PolyInterpolation class can transform a polynomial from the coefficient domain to the point domain and back in $O(d\log^{2}{d})$ time. So far it seems to work pretty well, but it still needs a little polishing up.

I also refactored the code I'd been using to compute invariant factors and wrapped it up in its own class called CoppersmithInvariantFactors. It was getting quite complicated and this should make it significantly easier to use for other applications. I also fixed some old Linbox bugs and cleaned up a matrix representation I'd been using for testing called sparse-map-map-matrix. It's now a fully fledged SparseMatrix type.

The plan now is to automatically extend the base field as needed when doing polynomial interpolation/evaluation so that there are sufficient points to represent the original polynomial. Once I've got that working I'll pull it all together into the determinant class and I'll be able to use that for the SNF computation. My original plan was to write some kind of a wrapper for the polynomial ring GivaroPoly but now I think I might be able to use a different SNF class (SmithFormIliopoulos) that already supports doing computations modulo $det(A)$.

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