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 A247086 #38 Jul 13 2024 02:39:57
%S A247086 2,12,24,90,234,528,588,1456,4320,4680,9024,11466,26208,35640,55104,
%T A247086 55552,90816,114660,121024,123648,185562,199584,297600,510720,674880,
%U A247086 2142720,2190336,4112640,4316928,5322240,5659641,7600320,8714160,10281600,12999168,15268368
%N A247086 Integers k such that numerator and denominator of sigma(k)/k are both prime.
%C A247086 2 is the only squarefree term in this sequence. - _Amiram Eldar_, Jul 13 2024
%H A247086 Amiram Eldar, <a href="/A247086/b247086.txt">Table of n, a(n) for n = 1..114</a> (terms 1..69 from Michel Marcus)
%e A247086 For k=2, sigma(k)/k = 3/2 with both numerator and denominator prime.
%t A247086 a247086Q[n_] := Block[{f = DivisorSigma[1, n]/n}, And[PrimeQ@ Numerator@ f, PrimeQ@ Denominator@ f]]; a247086[n_] := Select[Range@ n, a247086Q@ # &]; a247086[10^6] (* _Michael De Vlieger_, Jan 11 2015 *)
%o A247086 (PARI) isok(n) = my(ab = sigma(n)/n); isprime(numerator(ab)) && isprime(denominator(ab));
%Y A247086 Cf. A000203 (sigma(n)).
%Y A247086 Cf. A017665 (numerator of sigma(n)/n), A017666 (denominator of sigma(n)/n).
%K A247086 nonn
%O A247086 1,1
%A A247086 _Michel Marcus_, Jan 10 2015