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. ZAE Weiergewan, 11, rue Edmond Reuter, L-5326 Contern, LUXEMBOURG Office phone : +352-2454-2566 Office fax: +352-2454-3566 mobile phone: +49 151 52 40 66 44 e-mail: alexei.boulbitch at iee.lu<mailto:alexei.boulbitch at iee.lu>