ASKSAGE: Sage Q&A Forum - RSS feedhttps://ask.sagemath.org/questions/Q&A Forum for SageenCopyright Sage, 2010. Some rights reserved under creative commons license.Fri, 22 Mar 2013 17:15:40 +0100How do I evaluate sum() containing factorial()?https://ask.sagemath.org/question/9937/how-do-i-evaluate-sum-containing-factorial/I am trying to evaluate a sum containing a factorial, but need to do copy and paste of the interim result to get the final answer in the sage notebook (ver. 5.7). Is there a direct way?
var('i k n t')
sum(factorial(3-i)*k^i*t^i, i,0,n)(k=1, n=3, t=4)
> sum(4^i*factorial(-i + 3), i, 0, 3)
If I copy the result into a new input cell and evaluate:
sum(4^i*factorial(-i + 3), i, 0, 3)
only then I obtain the desired
>94
This is annoying, as I would like to compute the result for a long list of n and t and plot the results.
EDIT:
Maybe I simplifed the question too much. Just to specify again why I would like to use symbolics: I actually wanted to evaluate
var('i k n t')
sum(factorial(n-i)*k^i*t^i, i,0,n)
for different values of n and get the symbolic result, e.g. for n = 3, I would expect:
3*k^3*t^3 + 2*k^2*t^2 + k*t
**EDIT2:** Betrema's edited solution is very helpful:
[sum(factorial(n-i)*k^i*t^i, i, 0, n) for n in range(3)]
gives
[1, k*t + 1, k^2*t^2 + k*t + 2]
as desired. **The only remaining question is:
Why does sum(factorial(n-i)*k^i*t^i, i, 0, n)(n=3)
not give k^2*t^2 + k*t + 2?**
Does the .subs() method work differently on symbolic sums than on other symbolic equations?
Thanks again!
stanFri, 22 Mar 2013 17:15:40 +0100https://ask.sagemath.org/question/9937/