Puzzled by Sum
- To: mathgroup at smc.vnet.net
- Subject: [mg124570] Puzzled by Sum
- From: Themis Matsoukas <tmatsoukas at me.com>
- Date: Wed, 25 Jan 2012 07:03:32 -0500 (EST)
- Delivered-to: l-mathgroup@mail-archive0.wolfram.com
I am trying to define a function F[X] whose argument is the upper limit in a summation:
A[i_, j_] := i/j + j/i
F[X_] := (Sum[A[i - j, j], {j, 1, i - 1}]) /. i -> X
but when I evaluate, say F[2], I get
-2 DifferenceRoot[{\[FormalY],\[FormalN]}\[Function]{(\[FormalN]-2) \[FormalY](\[FormalN])+(3-2 \[FormalN]) \[FormalY](\[FormalN]+1)+(\[FormalN]-1) \[FormalY](\[FormalN]+2)==0,\[FormalY](0)==0,\[FormalY](1)==-(1/2)}][2]-1
where the result should have been 2:
ii = 2;
Sum[A[ii - j, j], {j, 1, ii - 1}]
2
If I use A[i,j]=i j or other simple functions, everything works as expected.
Themis
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