Re: problem with Sum
- To: mathgroup at smc.vnet.net
- Subject: [mg100684] Re: problem with Sum
- From: Bill Rowe <readnews at sbcglobal.net>
- Date: Thu, 11 Jun 2009 07:06:59 -0400 (EDT)
On 6/10/09 at 5:31 AM, olfa.mraihi at yahoo.fr (olfa) wrote:
>I have this input: Sum[a[k], {k, 1, j}] - Sum[a[k], {k, 1, i+j-n}]
>How it is possible to let mathematica gives as output: Sum[a[k], {k,
>i+j-n+1, j}] if i+j-n<j and Sum[a[k], {k, j+1, i+j-n}] if i+j-n>j
Have you looked at the documentation for If? Specifically,
If[i<n, Sum[a[k], {k, 1, i+j-n}], Sum[a[k], {k, j+1, i+j-n}]]
here, I've simplified your i+j-n < j to the equivalent i < n.
Or perhaps you are asking how to transform
Sum[a[k], {k, 1, j}] - Sum[a[k], {k, 1, i+j-n}]
to say
Sum[a[k], {k, 1, i+j-n}]
That can be done using pattern matching with a replacement rule.
That is:
In[9]:= x = Sum[a[k], {k, 1, j}] - Sum[a[k], {k, 1, i + j - n}]
Out[9]= Sum[a[k], {k, 1, j}] - Sum[a[k], {k, 1, i + j - n}]
In[10]:= First[x] /. j -> i + j - n
Out[10]= Sum[a[k], {k, 1, i + j - n}]