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

1,2

Links

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

Example

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

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 *)

Prog

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

Crossrefs

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

Sequence in context: A352393 A057830 A013916 * A324851 A348572 A334156

Adjacent sequences: A141110 A141111 A141112 * A141114 A141115 A141116

Keyword

nonn

Author

Leroy Quet, Jun 04 2008

Extensions

More terms from Emeric Deutsch, Jun 05 2008

Status

approved