Comment #7 on issue 2476 by [email protected]: nth order Derivative
http://code.google.com/p/sympy/issues/detail?id=2476
It doesn't make sense taking a derivative with respect to a number, or does
it?
The way I see it, we have:
d /d \|
--(f(x, y)) = |---(f(x', y))||
dx \dx' /|x'=x
In the rhs, x' is a dummy. So applying .subs(x, z) we arrive at
/d \|
|--(f(x, y))||
\dx /|x=z
and this happens to be expressable in simpler form as
d
--(f(z, y))
dz
So the variables in Derivative are used for 2 purposes: specifying in
respect to which variable the derivative should be taken, and at what point
the result should be evaluated. So that variable is a free symbol, but when
we represent it using Subs(), we see that inside the expression it can be
viewed as a dummy symbol, that'll be later substituted by the point, which
is a true symbol.
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