Updates:
Status: Accepted
Labels: Milestone-Release0.7.0 WrongResult
Comment #1 on issue 1938 by ronan.l...@gmail.com: Recursive trigsimp
destroys sums of derivative terms because cse fails on these sums
http://code.google.com/p/sympy/issues/detail?id=1938
I see what's happening:
cse transforms (virtually) diff(f(x,y),x)+diff(f(x,y),y) into (diff(x0, x)
+ diff(x0,
y)).subs(x0, f(x,y)) but diff(x0, x) is 0, so the result is 0.
I don't know how to fix this. cse should be made to recognise that f(x, y)
depends on
x and y and thus cannot be transformed into something that doesn't. Maybe
the easiest
thing to do would be to define x0 as an object for which diff(x0, x) remains
unevaluated.
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