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 <javascript:>>
> 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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