Comment #4 on issue 1525 by asmeurer: No integration by substitution
http://code.google.com/p/sympy/issues/detail?id=1525
I bisected it to
commit c20062e78c34a39096157de7c294db49dccc318b
Author: smichr <smi...@gmail.com>
Date: Thu Mar 31 21:58:54 2011 +0545
fix fdiff to use dummies
f(g(x)).diff(x) still applies the chain rule but the argument to
f is replaced with a dummy variable so as not to introduce a non-
symbol symbol (e.g. f(x)) of differentiation. Use of polys12's
Pure() rather than a dummy may be preferable.
>>> f(g(x)).diff(x)
D(f(_u), _u)*D(g(x), x)
Places where _eval_derivative was testing for the symbol of
differentiation were changed since that is handled in Derivatives
__new__ method. The __new__ docstring was updated to try make
it clear that _eval_derivative methods should not need to check
for this (and they shouldn't recursively call _eval_drivative
either).
I don't like this. I think the correct fix is issue 1620. And also, this
is plain wrong, because x is NOT a bound variable of Derivative(f(x), x)!
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