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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

Colin Barker, Table of n, a(n) for n = 1..1000

EXAMPLE

a(3) = 180 because the first 3 pentagonal numbers are [1,5,12] and 3/(1/1+1/5+1/12) = 180/77.

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 * A162136 A053363 A027572

Adjacent sequences:  A250324 A250325 A250326 * A250328 A250329 A250330

KEYWORD

nonn

AUTHOR

Colin Barker, Nov 18 2014

STATUS

approved

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Last modified June 25 10:09 EDT 2021. Contains 345453 sequences. (Running on oeis4.)