OFFSET
1,1
COMMENTS
No primes are in this sequence.
LINKS
Amiram Eldar, Table of n, a(n) for n = 1..10000 (terms 1..2500 from Colin Barker)
EXAMPLE
18 is a term because the nontrivial divisors of 18 are [2,3,6,9] and 4 / (1/2 + 1/3 + 1/6 + 1/9) = 18/5.
MATHEMATICA
Select[Range[235], CompositeQ[#] && Numerator[(DivisorSigma[0, #] - 2) * #/(DivisorSigma[1, #] - # -1)] == # &] (* Amiram Eldar, Mar 02 2020 *)
PROG
(PARI)
harmonicmean(v) = #v / sum(k=1, #v, 1/v[k])
nontrivialdivisors(n) = d=divisors(n); vector(#d-2, k, d[k+1])
s=[]; for(n=2, 500, t=nontrivialdivisors(n); if(#t>0 && numerator(harmonicmean(t))==n, s=concat(s, n))); s
CROSSREFS
KEYWORD
nonn
AUTHOR
Colin Barker, Nov 17 2014
STATUS
approved