But the point is that diff will take derivatives with respect to
functions just fine (or at least it's supposed to, that bug
notwithstanding). And then the sum will never evaluate, because it
can't be evaluated.
Aaron Meurer
On Tue, Jun 4, 2013 at 10:32 PM, Jeremy <lafl...@gmail.com> wrote:
> Thanks! and yes z_i and q_i would depend on i, but for the purposes of the
> derivation they can just be symbols.
>
>
> On Tuesday, June 4, 2013 11:23:49 PM UTC-4, Aaron Meurer wrote:
>>
>> 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 <laf...@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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>
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