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Characteristic function of A014613.
See A101638 for the inverse Moebius transform of this sequence.
Table of n, a(n) for n=1..100.
Index entries for characteristic functions
Eric Weisstein's World of Mathematics, Almost Prime.
a(100) = 1 because 100 = 2 * 2 * 5 * 5 is the product of exactly 4 primes and thus is a 4-almost prime.
(PARI) a(n)=bigomega(n)==4 \\ Charles R Greathouse IV, Jan 31 2017
Cf. A101638, A014613, A000040, A001358, A014612, A014614.
Sequence in context: A085980 A023974 A011730 * A011729 A011728 A133010
Adjacent sequences: A101634 A101635 A101636 * A101638 A101639 A101640
Jonathan Vos Post, Dec 10 2004