This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A247081 #19 Mar 02 2020 07:19:36 %S A247081 8,15,18,21,33,35,39,45,51,55,57,63,65,69,77,81,85,87,91,93,95,99,111, %T A247081 115,117,119,123,128,129,133,141,143,145,147,153,155,159,161,162,171, %U A247081 175,177,183,185,187,201,203,205,207,209,213,215,217,219,221,235 %N A247081 Positive integers k such that the numerator of the harmonic mean of the nontrivial divisors of k is equal to k. %C A247081 No primes are in this sequence. %H A247081 Amiram Eldar, <a href="/A247081/b247081.txt">Table of n, a(n) for n = 1..10000</a> (terms 1..2500 from Colin Barker) %e A247081 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. %t A247081 Select[Range[235], CompositeQ[#] && Numerator[(DivisorSigma[0, #] - 2) * #/(DivisorSigma[1, #] - # -1)] == # &] (* _Amiram Eldar_, Mar 02 2020 *) %o A247081 (PARI) %o A247081 harmonicmean(v) = #v / sum(k=1, #v, 1/v[k]) %o A247081 nontrivialdivisors(n) = d=divisors(n); vector(#d-2, k, d[k+1]) %o A247081 s=[]; for(n=2, 500, t=nontrivialdivisors(n); if(#t>0 && numerator(harmonicmean(t))==n, s=concat(s, n))); s %Y A247081 Cf. A250094, A250095. %K A247081 nonn %O A247081 1,1 %A A247081 _Colin Barker_, Nov 17 2014