## Update

It’s been a while since I’ve posted here, so I figured an update was in order. Here is a list of stuff that I have done since my last post.

– I recovered my data from my Terminal history. This wasn’t too difficult as I predicted. I just had to do some minor formatting on the git commit --interactive data to make it a valid git patch file. For whatever reason, a handful of the changes wouldn’t apply because git couldn’t find where changed lines were, even though they were identical to what was in the patch. git apply doesn’t seem to have a merge option, but eventually I found the --reject option, which puts failed patches in .rej files, instead of just failing the whole apply.

– I got separable equations implemented in dsolve. I actually did this on the road before I lost my data, but I failed to mention it before, so here it is. The hardest part with that was creating a decent separatevars() function that could separate just about any funciton. As I mentioned in an earlier post, this involved changing the way that SymPy handles automatic combining of exponents in the core, as well as refactoring expand. I also had to make the function completely independent of match, because match is too buggy to work correctly for separable equations.

– Speaking of refactoring expand and combining exponents, that work made it in! It is the first major thing that I have done that has actually made it into the main SymPy repo. It got in just before the release of SymPy 0.6.5-beta2, so it should be in the final release of SymPy 0.6.5. Most likely, none of my ODE stuff will make it in until 0.7.

– I started to work on Variation of Parameters, but before I could actually get to the variation of parameters part, I needed to be able to solve a homogeneous equation $a_ny^{(n)}+a_{n-1}y^{(n-1)}+\dots+a_1y'+a_0y=0$ ($a_i$ constant for all $i$). If you know how that works, it involves finding the roots of the polynomial given by $a_nr^n+a_{n-1}r^{n-1}+\dots+a_1r+a_0=0$. Depending on whether these roots are real, imaginary, or complex, you have different solutions with exponentials or sin’s and cos’s. I had no trouble getting the exponentials and the sin’s and cos’s to work correctly (SymPy already has a root finder that I put to work), but I did have a problem getting the arbitrary constants to work correcty. It turns out that the code for that would be much simpler if I had an arbitrary constant type that automatically “absorbed” other constants. Since I had planned on doing that anyway, I decided to put the rest of variation of parameters on hold and begin work on that.

– We had a Documentation day on June 30, and I decided to write up a document that would help people new to SymPy and Python with some of the gotchas and pitfalls. For example, unlike most other independent CAS’s like Maple, you can’t just type 1/2 in SymPy to get $\frac{1}{2}$. That is because Python evaluates it numerically. You have to do S(1)/2 or Rational(1,2) to get the Rational class. It’s all things like that. It’s taken me a while to get it together, not because it took me long to write it, but because it has to be in the Sphinx documentation format, which I have had to learn. I am just finishing it up now.

– I met with Ondrej on Saturday. He went down from Los Alamos to Carlsbad with a friend to see the caverns, and they stopped here in Albuquerque on the way back up. He came just in time to see the fireworks, and after that got some dinner. We weren’t able to do any coding, but hopefully we will be able to meet up again later this summer to do some of that.