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 A293275 #20 Oct 10 2017 08:17:57 %S A293275 0,1,2,3,0,4,0,3,1,2,6,2,5,3,4,4,5,3,9,6,0,6,9,2,12,9,9,4,0,9,5,8,5,1, %T A293275 12,6,16,12,2,7,18,8,3,4,0,10,8,5,16,2,8,16,14,10,6,1,2,8,3,18,12,8, %U A293275 18,11,11,4,14,9,5,10,6,11,5,5,16,7,14,12,8,13,9,18 %N A293275 Exponent of highest power of 2 dividing the n-th (2,k)-perfect number A019278. %H A293275 Michel Marcus, <a href="/A293275/b293275.txt">Table of n, a(n) for n = 1..130</a> %F A293275 a(n) = A007814(A019278(n)). %o A293275 (PARI) lista(nn) = for (n=1, nn, if (denominator(sigma(sigma(n))/n) == 1, print1(valuation(n, 2), ", "));); \\ _Michel Marcus_, Oct 10 2017 %Y A293275 Cf. A007814, A019278, A205597 (when a(n) is zero). %Y A293275 Cf. A134665 (analog for multiperfect numbers). %K A293275 nonn %O A293275 1,3 %A A293275 _Michel Marcus_, Oct 10 2017