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A250548 Numerator of the harmonic mean of the first n 9-gonal numbers. 2
1, 9, 216, 6624, 207000, 9190800, 825640200, 16041009600, 174445979400, 2519775258000, 8315258351400, 716624083375200, 33382738550561400, 1114469886995665200, 5970374394619635000, 340709365452960504000, 6878070315081640174500 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,2
LINKS
EXAMPLE
a(3) = 216 because the first 3 9-gonal numbers are [1,9,24], and 3/(1/1+1/9+1/24) = 216/83.
MATHEMATICA
Module[{nn=20, pns}, pns=PolygonalNumber[9, Range[nn]]; Numerator[Table[ HarmonicMean[Take[pns, n]], {n, nn}]]] (* Requires Mathematica version 10 or later *) (* Harvey P. Dale, Sep 30 2016 *)
PROG
(PARI)
harmonicmean(v) = #v / sum(k=1, #v, 1/v[k])
s=vector(30); for(n=1, #s, s[n]=numerator(harmonicmean(vector(n, k, (7*k^2-5*k)/2)))); s
CROSSREFS
Cf. A001106 (9-gonal numbers), A250549 (denominators).
Sequence in context: A188409 A109587 A067426 * A007108 A007107 A217042
KEYWORD
nonn,frac
AUTHOR
Colin Barker, Nov 25 2014
STATUS
approved

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Last modified April 18 02:55 EDT 2024. Contains 371767 sequences. (Running on oeis4.)