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%I #20 Oct 09 2017 00:04:19
%S 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,
%T 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,
%U 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
%N a(n) = 1 if n is a 4-almost prime, that is a product of exactly four (not necessarily distinct) primes, 0 otherwise.
%C Characteristic function of A014613.
%C See A101638 for the inverse Moebius transform of this sequence.
%H Antti Karttunen, <a href="/A101637/b101637.txt">Table of n, a(n) for n = 1..65537</a>
%H Eric Weisstein's World of Mathematics, <a href="http://mathworld.wolfram.com/Almostprime.html">Almost Prime.</a>
%H <a href="/index/Ch#char_fns">Index entries for characteristic functions</a>
%H <a href="/index/Eu#epf">Index entries for sequences computed from exponents in factorization of n</a>
%e a(100) = 1 because 100 = 2 * 2 * 5 * 5 is the product of exactly 4 primes and thus is a 4-almost prime.
%o (PARI) a(n)=bigomega(n)==4 \\ _Charles R Greathouse IV_, Jan 31 2017
%Y Cf. A101638, A014613, A000040, A001358, A014612, A014614.
%K easy,nonn
%O 1,1
%A _Jonathan Vos Post_, Dec 10 2004
%E Name edited by _Antti Karttunen_, Oct 08 2017
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