A101637 a(n) = 1 if n is a 4-almost prime, that is a product of exactly four (not necessarily distinct) primes, 0 otherwise.

0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 0, 1, 0, 0, 0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 1, 0, 0, 0, 1, 0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1

Offset: 1

Author

Jonathan Vos Post, Dec 10 2004

Keywords

• easy
• nonn

Comments

Characteristic function of A014613.

See A101638 for the inverse Moebius transform of this sequence.

Examples

			a(100) = 1 because 100 = 2 * 2 * 5 * 5 is the product of exactly 4 primes and thus is a 4-almost prime.
		

Links

• Antti Karttunen, Table of n, a(n) for n = 1..65537
• Eric Weisstein's World of Mathematics, Almost Prime.
• Index entries for characteristic functions
• Index entries for sequences computed from exponents in factorization of n

Crossrefs

Cf. A101638, A014613, A000040, A001358, A014612, A014614.

Programs

Mathematica

Table[If[PrimeOmega[n]==4,1,0],{n,100}] (* Harvey P. Dale, Sep 13 2024 *)

PARI

a(n)=bigomega(n)==4 \\ Charles R Greathouse IV, Jan 31 2017

Extensions

Name edited by Antti Karttunen, Oct 08 2017