Sage Sums

25 Feb 2021 - Tags: sage

Today I learned that sage can automatically simplify lots of sums for us by interfacing with maxima. I also learned recently that the init.sage file exists, which let me fix some minor gripes with my sage. Notably, I was able to add commands aa and nn which automatically get ascii art or a numeric answer for the most recent expression! This is going to be a short post just to highlight how these things work, since I had to figure them out for myself.

I was reading Concrete Mathematics the other night when I came across the section on Gosper’s Algorithm. This promises to solve a large class of sums (the hypergeometric ones) algorithmicaly, which is extremely alluring. I periodically find myself trying to simplify tricky sums (either for mse questions or for some problem I’m thinking about) and it would be nice to offload that thinking to a computer.

Unfortunately, googling around for “gosper’s method sage” and similar didn’t actually give anything useful (at least not quickly). In hindsight, it turns out there’s actually a gosper_sum built in (see the docs), but for some reason I didn’t find it at the time. After some searching I instead found a github repo that coded up gosper’s algorithm, as well as a bunch of other algorithms from a book. This was how I found my way to Petkovsek, Wilf, and Zeilberger’s A=B, which has lots of similar algorithms for algorithmically simplifying sums. It’s an extremely interesting read, both mathematically and historically, and I’ve been enjoying it so far.

Before I learned it was built-in after all, I was planning to put up a blog post with an implementation of gosper’s algorithm so that people could come here to simplify their sums. Thankfully, I did eventually find the implementation, which saved me a bunch of coding! Sage actually aliases over python’s default sum function, and will pass off symbolic sums to maxima where they’re evaluated using tons of powerful techniques (one of which is gosper’s algorithm). The reason I was having trouble finding a function for this was (in part) because it’s baked into the sum function already!

I’m still putting up this post, in part to share this realization with anyone else who didn’t know about it (which probably isn’t many people…), but in part to still provide a place where people can come to simplify sums. In case you don’t have a sage installation on your own computer, you can modify one of the examples below and evaluate your favorite summation here!

Let’s see some quick examples. The syntax sum(f,k,a,b) corresponds to $\sum_{k=a}^b f$.

This can solve fairly complex sums. These come from an exercise in A=B (exercise 1d and 1e from chapter 5). After seeing the solutions, I’m definitely glad I didn’t have to work them out by hand!

This has already helped me “in the wild”. There is an mse question which asked about the sum $\sum_{n=0}^\infty \frac{16n^2 + 20n + 7}{(4n+2)!}$. A commenter asks whether OP wants a closed form, or merely a convergence result. The sum certainly looks like it doesn’t admit a nice closed form, but I’ve been deceived before. Instead of wasting a few minutes trying to find a nice closed form (which is what I would have done even a few days ago), we can simply ask sage:

Sage happily computed a closed form for this sum… It just happens to use a bunch of hypergeometric functions! This pretty quickly answers the “does OP want a closed form” question, assuming OP’s professor isn’t a sadist¹.

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As an aside, in the above example we computed something exactly, but then used .n() in order to get a numerical value (which is often better for getting a sense of things). Since sage will let you write _ to get the output of the last command, _.n() is probably my most typed command. Either that or ascii_art(_), which draws an ascii version of whatever your most recent output was. Since I use sage in a terminal, rather than a jupyter notebook, this saves me endless parsing related headaches when it comes to actually reading sage’s output.

If you also find yourself using these commands all the time, I can’t recommend the following aliases enough. These are part of my init.sage, and have changed my life. If you want to see my entire sage configuration, you can find it (with the rest of my dotfiles) here.

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Before I end this post, there are a few more parting observations that I want to squeeze in.

First, sage can solve recurrences for you as well, by using either maxima’s solve_rec or sympy’s rsolve. I have a wrapper in my init.sage that makes using the latter slightly more convenient.

Second, if you’re faced with a particularly stubborn sum that sage won’t simplify for you, you should try using maxima directly. You can actually do this from inside sage by using maxima.console() and then loading the simplify_sum package. You can see a worked out example here, and you can see all the high-power tools that simplify_sum buys you here.

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