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MathGroup Archive 2004

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Re: Defining a function

  • To: mathgroup at smc.vnet.net
  • Subject: [mg49999] Re: Defining a function
  • From: koopman at sfu.ca (Ray Koopman)
  • Date: Tue, 10 Aug 2004 06:02:57 -0400 (EDT)
  • References: <cen7rv$h8$1@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

"PSi" <psinome at tee.gr> wrote in message news:<cen7rv$h8$1 at smc.vnet.net>...
> I want to compute the Sum[a[i, j]] as a function of k when
> 1 <= i <= m,  1 <= j <= n and i+j=k. The numbers m, n and
> the function a[i, j] are given. Is it possible to define such a
> function with Mathematica 4.1?
> Thanks, PSi

    j=k-i        : from i+j=k
 1 <= k-i <= n   : substitute k-i for j
-1 >= i-k >= -n  : multiply by -1
k-1 >= i >= k-n  : add k
k-n <= i <= k-1  : restate

f[k_] := Sum[a[i,k-i], {i, Max[1,k-n], Min[m,k-1]}]


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