A250548 Numerator of the harmonic mean of the first n 9-gonal numbers.

1, 9, 216, 6624, 207000, 9190800, 825640200, 16041009600, 174445979400, 2519775258000, 8315258351400, 716624083375200, 33382738550561400, 1114469886995665200, 5970374394619635000, 340709365452960504000, 6878070315081640174500

Offset

1,2

Links

Colin Barker, Table of n, a(n) for n = 1..800

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

Adjacent sequences: A250545 A250546 A250547 * A250549 A250550 A250551

Keyword

nonn,frac

Author

Colin Barker, Nov 25 2014

Status

approved