A050436 Third-order composites.

16, 21, 25, 26, 28, 33, 36, 38, 39, 42, 48, 49, 50, 52, 55, 56, 57, 60, 64, 68, 69, 70, 72, 74, 77, 78, 80, 84, 87, 88, 90, 93, 94, 95, 98, 100, 104, 105, 106, 110, 111, 115, 117, 118, 119, 121, 122, 124, 125, 126, 130, 133, 135, 138, 140, 141, 145, 146, 147

Offset: 1

Author

Michael Lugo (mlugo(AT)thelabelguy.com), Dec 22 1999

Keywords

• easy
• nonn

Examples

			C(C(C(8))) = C(C(15)) = C(25) = 38. So 38 is in the sequence.
		

Links

• N. Fernandez, An order of primeness, F(p)
• N. Fernandez, An order of primeness [cached copy, included with permission of the author]

Crossrefs

Cf. A049076, A049077, A049078, A049079, A049080, A049081, A006450, A050435, A050438, ...

Programs

Maple

C := remove(isprime,[$4..1000]): seq(C[C[C[C[n]]]],n=1..100);

Mathematica

Nest[Values@ KeySelect[MapIndexed[First@ #2 -> #1 &, #], CompositeQ] &, Select[Range@ 150, CompositeQ], 2] (* Michael De Vlieger, Jul 22 2017 *)

Formula

Let C(n) be the n-th composite number, with C(1)=4. Then these are numbers C(C(C(n))).

Extensions

More terms from Asher Auel Dec 15 2000