Articles

Status Update—Week 7

In Uncategorized on July 8, 2011 by nessgrh

I’m starting to get into a phase were results of my new code are not as impressive as one might wish. Basically I hit one obscure integral that doesn’t work, and then I spend hours hunting down some bugs to fix it. These bugs can be minor typos or rather deep conceptual problems (see the branch cuts post), but nonetheless they are not very user-visible.

In any case, this week I fixed the combsimp algorithm for gamma functions, I implemented most of the expansion algorithm for G functions, I added classes to represent unevaluated transforms (as suggested in a comment to my last blog post), and I just about started to to extend the lookup tables. The last part turns out to be a real stress test for my code (since obviously we want to derive the formulas wherever possible, instead of putting them all into tables).

As I said before, there is not a lot to show off. Perhaps one word of clarification: there was code to expand meijer g-functions all the time. It works by evaluating the defining integral(s) using the residue theorem, i.e. using the (generalised) slater theorem. However, in some cases this does not work, because the series are properly non-hypergeometric. This can be quite annoying. Consider, for example G_{1, 2}^{2, 1}\left( y \atop {0, y} \middle| x \right) = e^{x} \Gamma\left(1 - y \right) \Gamma(y, x). This formula holds for all y that are not positive integers (that is for all y so that the G-function on the left hand side makes sense). But for negative integers, slater’s theorem does not apply! This is quite a shame, since obviously substituting them in the end result is no problem at all, there does not seem to be a limit involved. But this is not the case, there is some gamma function cancellation going on. In fact, whenever Slater’s theorem applies, we know that the G-function is a sum of terms of the form z^a F(z), where F(z) is unbranched at the origin. But \Gamma(-n, z) has logarithmic monodromy at the origin. Thus there really is a limiting process.

Hence I spent about 1.5 days developing this new code. And then another half a day thinking about the branch cuts. Let’s see how much longer it is going to take until there is a satisfactory solution ^^.

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3 Responses to “Status Update—Week 7”

  1. > Basically I hit one obscure integral that doesn’t work, and then I spend hours hunting down some bugs to fix it.

    That sounds exactly like what I do with the Risch code all the time. 🙂

  2. We all have slow/difficult weeks, but to tell the truth I’m impressed with how much you’ve got done so far. Keep up the good work – I’m enjoying reading about your progress.

  3. Thanks for (both of) your encouraging words. I’ll try to make the best of it. 🙂

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