%I #17 Feb 19 2019 04:42:23

%S 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 64,68,69,72,76,80,84,87,88,90,92,93,96,104,108,111,112,116,120,123,

%U 124,126,128,129,132,136,141,144,148,150,152,156,159,164,172,176,177,180

%N Positive integers k such that d(d(k)) divides k, where d(k) is the number of divisors of k.

%H Jinyuan Wang, <a href="/A141113/b141113.txt">Table of n, a(n) for n = 1..10000</a>

%e 28 has 6 divisors and 6 has 4 divisors. 4 divides 28, so 28 is in the sequence.

%p 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 Select[Range[200],Divisible[#,DivisorSigma[0,DivisorSigma[0,#]]]&] (* _Harvey P. Dale_, Feb 05 2012 *)

%o (PARI) is(k) = k%numdiv(numdiv(k)) == 0; \\ _Jinyuan Wang_, Feb 19 2019

%Y Cf. A000005 (tau), A010553, A141114, A141115.

%K nonn

%O 1,2

%A _Leroy Quet_, Jun 04 2008

%E More terms from _Emeric Deutsch_, Jun 05 2008