Meeting Ondrej in Los Alamos

So Ondrej was kind enough to have me over to his house in Los Alamos for Friday and Saturday. We spent a lot of time coding together. While we worked on some other things too, we mainly worked on my constant class. Ondrej and I came up with an idea for restructured Mul and Add classes that would allow different objects in them to handle the other objects in them. The way it is now, if I want my Constant object to absorb other objects, like 2*a*C*x => C*x, I have to hardcode it into Mul. The same is true with Add. This makes the Mul and Add classes muddy. Right now, there is already special handling for other such dynamic objects such as Order classes (e.g., O(x)), and Infinity class. See the Add.flatten() and Mul.flatten() methods in sympy/core/add.py and sympy.core/mul.py to see what I mean.

We came up with a system where symbols and numbers are handled the same way, because we need them to be fast, but if an expression has an object that has a handle_mul() method, it will call that method with the other objects in the expression in the Mul/Add and the object will take care of the special handling. Ondrej was able to get most of it working in his experimental core that doesn’t use assumptions here. We will hopefully end up using it, but we need to wait until we merge the new assumptions system.

So in the mean while, I have a working Constant class that modifies Mul and Add here. Since it will take a while until the new assumptions system is done (Fabian Seoane is doing it for his Google Summer of Code project), we may end up temporarily adding in my Constant branch. Once I have a working Constant class, I can solve homogeneous differential equations (not to be confused with first order differential equations with homogeneous coefficients) with one case (there are several cases depending on whether the roots of the so called characteristic equation are real, imaginary, or complex, but we can actually handle them in one case if we have constants that combine into each other. More on this in a later post). Once I have that (which I have everything already except for the arbitrary constants), I can then implement variation of parameters, which, along with separable, will probably be the most used solver of the ones that I will implement this summer.

Once I get those, I can clean up a lot of the 2nd order differential equation code in dsolve (currently it is just a hack with a bunch of special cases all covered by variation of parameters). With that code cleaned up, I can refactor dsolve to use my proposed hints engine, which will allow the user to choose which methods they want to use to solve an equation (more on that in a later post too).

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