A250132 Numerator of the harmonic mean of the first n composite numbers.

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

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