A141113 Positive integers k such that d(d(k)) divides k, where d(k) is the number of divisors of k.

1, 2, 4, 6, 12, 15, 16, 20, 21, 24, 27, 28, 32, 33, 36, 39, 40, 44, 48, 51, 52, 56, 57, 60, 64, 68, 69, 72, 76, 80, 84, 87, 88, 90, 92, 93, 96, 104, 108, 111, 112, 116, 120, 123, 124, 126, 128, 129, 132, 136, 141, 144, 148, 150, 152, 156, 159, 164, 172, 176, 177, 180

Offset: 1

Author

Leroy Quet, Jun 04 2008

Keywords

• nonn

Examples

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

Links

• Jinyuan Wang, Table of n, a(n) for n = 1..10000

Crossrefs

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

Programs

Maple

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

Mathematica

Select[Range[200],Divisible[#,DivisorSigma[0,DivisorSigma[0,#]]]&] (* Harvey P. Dale, Feb 05 2012 *)

PARI

is(k) = k%numdiv(numdiv(k)) == 0; \\ Jinyuan Wang, Feb 19 2019

Extensions

More terms from Emeric Deutsch, Jun 05 2008