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A250327
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Numerator of the harmonic mean of the first n pentagonal numbers.
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3
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1, 5, 180, 2640, 23100, 157080, 183260, 2408560, 317026710, 10215305100, 89894684880, 19613385792, 403708857552, 17825298787296, 8681152006800, 435215087274240, 2312080151144400, 43249499297877600, 45652249258870800, 2835244953971976000, 4394629678656562800
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OFFSET
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1,2
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LINKS
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Colin Barker, Table of n, a(n) for n = 1..1000
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EXAMPLE
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a(3) = 180 because the first 3 pentagonal numbers are [1,5,12] and 3/(1/1+1/5+1/12) = 180/77.
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PROG
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(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
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CROSSREFS
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Cf. A250328 (denominators).
Sequence in context: A136143 A035791 A324090 * A162136 A053363 A027572
Adjacent sequences: A250324 A250325 A250326 * A250328 A250329 A250330
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KEYWORD
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nonn
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AUTHOR
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Colin Barker, Nov 18 2014
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STATUS
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approved
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