Comment #11 on issue 1816 by [email protected]: Adding partial
derivatives and taking derivatives with respect to functions
http://code.google.com/p/sympy/issues/detail?id=1816
I don't understand what Derivative(f(g(x)), g(x)) or diff(f(g(x)), g(x))
are supposed to mean, mathematically. This looks like a return to the
situation before we straightened out issue 1620.
Also, this:
In [2]: diff(f(x), x).diff(f(x))
Out[2]: 0
seems wrong. diff(f(x), x) corresponds to a nonzero continuous linear
functional of f, so its derivative wrt. f can't be 0. The correct answer in
functional calculus is (δf'/δf(y))(x) = DiracDelta'(x - y).
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