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A250132 Numerator of the harmonic mean of the first n composite numbers. 2
4, 24, 72, 288, 1800, 2160, 17640, 4032, 9072, 3360, 18480, 20160, 240240, 258720, 1386000, 19219200, 183783600, 194594400, 205405200, 432432000, 454053600, 8086478400, 1207720800, 1260230400, 24942060000, 25939742400, 26937424800, 195545750400, 202529527200 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

LINKS

Colin Barker, Table of n, a(n) for n = 1..500

EXAMPLE

a(3) = 72 because the first 3 composite numbers are [4,6,8] and 3 / (1/4+1/6+1/8) = 72/13.

MATHEMATICA

Module[{comps=Select[Range[50], CompositeQ], len}, len=Length[comps]; Table[ HarmonicMean[ Take[comps, n]], {n, len}]]//Numerator (* Harvey P. Dale, Jun 30 2017 *)

PROG

(PARI)

harmonicmean(v) = #v / sum(k=1, #v, 1/v[k])

composite(n) = for(k=0, primepi(n), isprime(n++)&&k--); n \\ from A002808

s=vector(100); for(k=1, #s, s[k]=numerator(harmonicmean(vector(k, i, composite(i))))); s

CROSSREFS

Cf. A250133 (denominators).

Sequence in context: A261256 A011915 A199904 * A025220 A112742 A158494

Adjacent sequences:  A250129 A250130 A250131 * A250133 A250134 A250135

KEYWORD

nonn

AUTHOR

Colin Barker, Nov 14 2014

STATUS

approved

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Last modified October 19 13:01 EDT 2019. Contains 328222 sequences. (Running on oeis4.)