I'd consider this to be a bug. I'm sure a call to doit() was added in
there to make Integral(x, x).diff(x) return just x, but it's clearly
annoying.
I'm assuming you mean for q_i and z_i to mean that these things depend
on i. One way would be to actually use a function. Unfortunately,
there seem to be another bug:
In [5]: z_i = Function('z')(i)
In [6]: q_i = Function('q')(i)
In [7]: a_k = Sum(-q_i*z_i/k,(i,1,m))
In [8]: print a_k.diff(z_i)
0
I opened https://code.google.com/p/sympy/issues/detail?id=3867 and
https://code.google.com/p/sympy/issues/detail?id=3868 for these.
Aaron Meurer
On Tue, Jun 4, 2013 at 10:11 PM, Jeremy <lafl...@gmail.com> wrote:
> Hello,
>
> I am a new sympy user and am trying to work out a derivation using IPython3
> in a notebook.
>
>
> I have an expression:
> a_k = Sum(-q_i*z_i/k,(i,1,m)).
>
> I want a_k.diff(z_i), however the Sum gets evaluated and I get m*(otherwise
> correct stuff), and of course the sum goes away.
>
> I thought that using Sum() instead of summation(), the expression would not
> evaluate. I guess .diff() causes the expression to evaluate? Is there a way
> around this problem?
>
> If it was this one expression I would just make temp = stuff_inside_sum and
> .diff() that, but it is nested in yet another Sum(). Eg. phi =
> Sum(a_k/x,(k,1,oo)).
>
> Any help would be appreciated,
> Thanks!
>
>
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