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

1,1

Comments

Characteristic function of A014613.

See A101638 for the inverse Moebius transform of this sequence.

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

Example

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

Mathematica

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

Prog

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

Crossrefs

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

Sequence in context: A380397 A295884 A380454 * A337380 A011729 A297044

Adjacent sequences: A101634 A101635 A101636 * A101638 A101639 A101640

Keyword

easy,nonn

Author

Jonathan Vos Post, Dec 10 2004

Extensions

Name edited by Antti Karttunen, Oct 08 2017

Status

approved