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Re: Symbolic summation

  • To: mathgroup at
  • Subject: [mg101608] Re: [mg101561] Symbolic summation
  • From: Jaebum Jung <jaebum at>
  • Date: Fri, 10 Jul 2009 23:24:44 -0400 (EDT)
  • References: <>

Luca wrote:
> Hello all, I have a problem, I've to compute a symbolic summation, which takes a very long time if I have to do it by hand. I have to write the terms of the Volterra series, related to the associated linear equation of a system.
> The problem is that I'm not really confident with mathematica, I've tried to use symbolic calculations with sums, but I think that in my case is not so easy.
> The problem is that I want a solution of this form:
> x1+x2+x3+x4+x5...
> Where the 1, 2, 3 and 4 and 5 are the summation subscripts...
> My summation is not infinite, I have to stop it.
> If I write something like this:
> n = 3;
> Sum[kl, {l, 2, n}]
> I obtain 2kl, instead I want to obtain k2+k3.
> Is it possible with mathematica?

kl regarded as one symbol.   one way is using k[l] instead of kl and 
then convert it to kl.   For example,

tosymbol[s_[t_]] := ToExpression[ToString[Row[{s, t}]]]

In[490]:= Sum[k[l],{l,2,n}]
Out[490]= k[2]+k[3]
In[491]:= tosymbol/@%
Out[491]= k2+k3

In[492]:= Sum[tosymbol@k[l],{l,2,n}]

Out[492]= k2+k3

In[493]:= Sum[k[l],{l,2,n}]/.k[x_]:>tosymbol[k[x]]
Out[493]= k2+k3

- Jay

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