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a(n) = 1 if n is a product of exactly 3 (not necessarily distinct) primes, otherwise 0.
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%I #40 Jun 30 2020 05:37:03

%S 0,0,0,0,0,0,0,1,0,0,0,1,0,0,0,0,0,1,0,1,0,0,0,0,0,0,1,1,0,1,0,0,0,0,

%T 0,0,0,0,0,0,0,1,0,1,1,0,0,0,0,1,0,1,0,0,0,0,0,0,0,0,0,0,1,0,0,1,0,1,

%U 0,1,0,0,0,0,1,1,0,1,0,0,0,0,0,0,0,0,0,0,0,0,0,1,0,0,0,0,0,1,1,0

%N a(n) = 1 if n is a product of exactly 3 (not necessarily distinct) primes, otherwise 0.

%H Antti Karttunen, <a href="/A101605/b101605.txt">Table of n, a(n) for n = 1..10000</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>

%F a(n) = 1 if n has exactly three prime factors (not necessarily distinct), else a(n) = 0. a(n) = 1 if n is an element of A014612, else a(n) = 0.

%F a(n) = floor(Omega(n)/3) * floor(3/Omega(n)). - _Wesley Ivan Hurt_, Jan 10 2013

%e a(28) = 1 because 28 = 2 * 2 * 7 is the product of exactly 3 primes, counted with multiplicity.

%p A101605 := proc(n)

%p if numtheory[bigomega](n) = 3 then

%p 1;

%p else

%p 0;

%p end if;

%p end proc: # _R. J. Mathar_, Mar 13 2015

%t Table[Boole[PrimeOmega[n] == 3], {n, 100}] (* _Jean-François Alcover_, Mar 23 2020 *)

%o (PARI) is(n)=bigomega(n)==3 \\ _Charles R Greathouse IV_, Apr 25 2016

%Y Cf. A010051, A064911, (char funct. of) A014612, A101637, A123074.

%K easy,nonn

%O 1,1

%A _Jonathan Vos Post_, Dec 09 2004

%E Description clarified by _Antti Karttunen_, Jul 23 2017