Comment #14 on issue 1525 by asmeurer: No integration by substitution http://code.google.com/p/sympy/issues/detail?id=1525
They don't even have the same derivatives. (f(x).diff(x)).diff(x) == Derivative(f(x), x, x), whereas, (f(y).diff(y)).diff(x) == 0.
And of course the differentiation variable belongs in the free symbols, because it remains after the calculation in general:
In [62]: sin(x).free_symbols Out[62]: set(x) In [63]: Derivative(sin(x), x).free_symbols Out[63]: set(x) In [65]: Derivative(sin(x), x).doit().free_symbols Out[65]: set(x) -- You received this message because you are subscribed to the Google Groups "sympy-issues" group. To post to this group, send email to sympy-issues@googlegroups.com. To unsubscribe from this group, send email to sympy-issues+unsubscr...@googlegroups.com. For more options, visit this group at http://groups.google.com/group/sympy-issues?hl=en.
