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I'm having trouble plotting the result of FindDiferrenceRoot[], for following where a->(2/3) and n from 0 to 20:

FindDifferenceRoot[Sum[(2^k*(2*k)!*HermiteH[-(2*k)-1, 
                   a/Sqrt[2]]*((-1)^k*n!)/(k!^2*(n - k)!))/(2*k + 1), {k, 0, n}]]

Please advise.

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closed as too localized by Oleksandr R., acl, rm -rf, Sjoerd C. de Vries Aug 6 '12 at 21:22

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I suspect the main problem here is that FindDifferenceRoot isn't a Mathematica function (the DifferenceRoot is actually being produced by Sum). Given that both of your questions so far demonstrate incorrect function usage and have only admitted answers that guess at what you really meant to do, I'd ask you to please consider what it is you're asking (and whether the answers will be useful to others in future) and, by a careful reading of the documentation, try to identify why problems might have arisen before posting further questions. Thanks for your consideration! –  Oleksandr R. Aug 3 '12 at 12:29
    
(As it is, I'm voting to close this question, since you already got your answer, and, given the nonexistence of FindDifferenceRoot, it seems exceedingly unlikely that anyone else could be benefited by keeping this around for reference.) –  Oleksandr R. Aug 3 '12 at 12:36
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what is FindDifferenceRoot? –  acl Aug 3 '12 at 14:58
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1 Answer 1

up vote 2 down vote accepted
a[n_, k_] := a[n, k] = (-1)^k n!/((k!)^2 (n - k)!)
r[n_, b_] := Sum[2^k (2 k)! a[n, k] HermiteH[-2 k - 1, b/Sqrt[2]], {k, 0, n}]

ListLinePlot[Table[r[i, 2/3.], {i, 0, 20}], PlotRange -> Full]

Mathematica graphics

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