OFFSET
1,2
COMMENTS
a(n+1) is, for n >= 0, also the numerator of the partial sums of the reciprocal of the positive decagonal numbers A001107(n+1) with the denominators A294515(n) (provided A294515(n) = A250550(n+1)/(n+1)). - Wolfdieter Lang, Nov 02 2017
LINKS
Colin Barker, Table of n, a(n) for n = 1..850
FORMULA
a(n) = denominator(r(n)) with the rationals r(n) = n/Sum_{k=1..n} A001107(n), n >= 1. See the name. - Wolfdieter Lang, Nov 02 2017
EXAMPLE
a(3) = 307 because the first 3 positive decagonal numbers A001107 are [1,10,27], and 3/(1/1+1/10+1/27) = 810/307.
MATHEMATICA
With[{s = Array[PolygonalNumber[10, #] &, 19]}, Denominator@ Array[HarmonicMean@ Take[s, #] &, Length@ s]] (* Michael De Vlieger, Nov 02 2017 *)
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, (8*k^2-6*k)/2)))); s
CROSSREFS
KEYWORD
nonn,frac,easy
AUTHOR
Colin Barker, Nov 25 2014
STATUS
approved