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 A216780 #24 Jun 24 2014 03:19:05
%S A216780 1,9,10,12,25,26,34,44,49,56,58,74,76,81,82,90,106,120,121,122,146,
%T A216780 169,172,178,184,194,202,216,218,225,226,234,236,260,268,274,289,298,
%U A216780 300,306,312,314,332,340,346,361,362,386,394,396,408,428,440,441,458
%N A216780 Numbers n such that numerator(sigma(n)/n) and denominator(sigma(n)/n) are both odd.
%C A216780 a(n) contains odd squares (A016754), 3-perfect numbers (A005820) and 5-perfect numbers (A046060).
%C A216780 This is also the sequence of numbers x such that A243473(x) is even. - _Michel Marcus_, Jun 06 2014
%H A216780 Vincenzo Librandi, <a href="/A216780/b216780.txt">Table of n, a(n) for n = 1..1000</a>
%e A216780 sigma(10)/10 = 9/5; both 9 and 5 are odd, so 10 is in the sequence.
%t A216780 Select[Range[500], OddQ[Numerator[DivisorSigma[1, #]/#]] && OddQ[Denominator[DivisorSigma[1, #]/#]] &] (* _Alonso del Arte_, Sep 16 2012 *)
%o A216780 (PARI) ooab(k) = {for (i=1, k, ab = sigma(i)/i; if ((numerator(ab) % 2 == 1) && (denominator(ab) % 2 == 1), print1(i, ", ")););}
%Y A216780 Cf. A216781, A216782, A016754, A005820, A046060.
%K A216780 nonn,easy
%O A216780 1,2
%A A216780 _Michel Marcus_, Sep 16 2012