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 A141113 #17 Feb 19 2019 04:42:23 %S A141113 1,2,4,6,12,15,16,20,21,24,27,28,32,33,36,39,40,44,48,51,52,56,57,60, %T A141113 64,68,69,72,76,80,84,87,88,90,92,93,96,104,108,111,112,116,120,123, %U A141113 124,126,128,129,132,136,141,144,148,150,152,156,159,164,172,176,177,180 %N A141113 Positive integers k such that d(d(k)) divides k, where d(k) is the number of divisors of k. %H A141113 Jinyuan Wang, <a href="/A141113/b141113.txt">Table of n, a(n) for n = 1..10000</a> %e A141113 28 has 6 divisors and 6 has 4 divisors. 4 divides 28, so 28 is in the sequence. %p A141113 with(numtheory): a:=proc(n) if `mod`(n, tau(tau(n))) = 0 then n else end if end proc: seq(a(n),n=1..200); # _Emeric Deutsch_, Jun 05 2008 %t A141113 Select[Range[200],Divisible[#,DivisorSigma[0,DivisorSigma[0,#]]]&] (* _Harvey P. Dale_, Feb 05 2012 *) %o A141113 (PARI) is(k) = k%numdiv(numdiv(k)) == 0; \\ _Jinyuan Wang_, Feb 19 2019 %Y A141113 Cf. A000005 (tau), A010553, A141114, A141115. %K A141113 nonn %O A141113 1,2 %A A141113 _Leroy Quet_, Jun 04 2008 %E A141113 More terms from _Emeric Deutsch_, Jun 05 2008