Monday, 21 July 2014

Week of 7/14 Update

This week I focused on speeding up computations over GF(3). We have a number of matrices over GF(3) for which we want to compute the invariant factors. Fortunately, Bryan Youse has written a very efficient representation of these matrices called Sliced3. It uses 2 bits to represent each matrix element and operates over n/2 elements in parallel when computing a matrix-matrix multiplication where n is the word size. The code has been developed in parallel with the mainline Linbox source so the interface has diverged a bit from the rest of the Linbox matrix classes. Also, in order to speed up the critical applyLeft() method I had to specialize for this representation so that I use iterators directly rather than constructing temporary submatrices.

I further improved what I'm now calling my Pascal blackbox class which represents matrices of the form $A_{i,j}=c_{i+j}{i+j\choose{}j}$ with elements in GF(3). It turns out it's not necessary to be fast when computing ${n\choose{}k}$, instead you can take advantage of the fractal structure of this matrix. A $3^{n}\times{}3^{n}$ matrix $B_{i,j}={i+j\choose{}j}$ consists of 9 $3^{n-1}\times{}3^{n-1}$ submatrices, $B_{i,j}$ where $B_{1,1}=B_{1,2}=B_{1,3}=B_{2,1}=B_{3,1}$, $B_{2,2}=-A_{1,1}$ and $A_{2,3}=A_{3,3}=A_{3,2}=0$. This gives rise to a natural recursive implementation of applyLeft() which proves to be about 10 times fast when used with the Sliced3 package than my old generic sparse matrix implementation. It also only uses $O(n)$ memory instead of $O(n^{\frac{4}{3}})$

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