A250549 Denominator of the harmonic mean of the first n 9-gonal numbers.

1, 5, 83, 1945, 49177, 1833349, 141933773, 2422703191, 23493728113, 306193780933, 920490263449, 72844719575371, 3136916685865153, 97366698355411543, 487364191723246127, 26098860817171249607, 496295891513328449033, 1821113866092559163621

Offset

1,2

Links

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

Example

a(3) = 83 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, p9}, p9=PolygonalNumber[9, Range[nn]]; Table[Denominator[ HarmonicMean[ Take[p9, n]]], {n, nn}]] (* Requires Mathematica version 10 or later *) (* Harvey P. Dale, Aug 26 2019 *)

Prog

(PARI)
harmonicmean(v) = #v / sum(k=1, #v, 1/v[k])
s=vector(30); for(n=1, #s, s[n]=denominator(harmonicmean(vector(n, k, (7*k^2-5*k)/2)))); s

Crossrefs

Cf. A001106 (9-gonal numbers), A250548 (numerators).

Sequence in context: A173876 A368080 A348791 * A035512 A054953 A215172

Adjacent sequences: A250546 A250547 A250548 * A250550 A250551 A250552

Keyword

nonn,frac

Author

Colin Barker, Nov 25 2014

Status

approved