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Re: Puzzled by Sum

  • To: mathgroup at smc.vnet.net
  • Subject: [mg124618] Re: Puzzled by Sum
  • From: Alexei Boulbitch <Alexei.Boulbitch at iee.lu>
  • Date: Fri, 27 Jan 2012 06:07:21 -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





Try this:

F[i_] := (Sum[(i - j)/j + j/(i - j), {j, 1, i - 1}]);



F[2]



2



Alexei BOULBITCH, Dr., habil.

IEE S.A.

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e-mail: alexei.boulbitch at iee.lu<mailto:alexei.boulbitch at iee.lu>








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