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A250327 Numerator of the harmonic mean of the first n pentagonal numbers. 3
1, 5, 180, 2640, 23100, 157080, 183260, 2408560, 317026710, 10215305100, 89894684880, 19613385792, 403708857552, 17825298787296, 8681152006800, 435215087274240, 2312080151144400, 43249499297877600, 45652249258870800, 2835244953971976000, 4394629678656562800 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,2
LINKS
EXAMPLE
a(3) = 180 because the first 3 pentagonal numbers are [1,5,12] and 3/(1/1+1/5+1/12) = 180/77.
MATHEMATICA
Table[Numerator[n/Total[1/PolygonalNumber[5, Range[n]]]], {n, 30}] (* Harvey P. Dale, Jun 01 2022 *)
PROG
(PARI)
harmonicmean(v) = #v / sum(k=1, #v, 1/v[k])
s=vector(30); for(k=1, #s, s[k]=numerator(harmonicmean(vector(k, i, (3*i^2-i)/2)))); s
CROSSREFS
Cf. A250328 (denominators).
Sequence in context: A136143 A035791 A324090 * A366342 A162136 A053363
KEYWORD
nonn
AUTHOR
Colin Barker, Nov 18 2014
STATUS
approved

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Last modified May 29 22:36 EDT 2024. Contains 372954 sequences. (Running on oeis4.)