1,1

Antti Karttunen, Table of n, a(n) for n = 1..10000

Eric Weisstein's World of Mathematics, Almost Prime.

Index entries for characteristic functions

Index entries for sequences computed from exponents in factorization of n

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.

a(n) = floor(Omega(n)/3) * floor(3/Omega(n)). - Wesley Ivan Hurt, Jan 10 2013

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

A101605 := proc(n)

if numtheory[bigomega](n) = 3 then

1;

else

0;

end if;

end proc: # R. J. Mathar, Mar 13 2015

Table[Boole[PrimeOmega[n] == 3], {n, 100}] (* Jean-François Alcover, Mar 23 2020 *)

(PARI) is(n)=bigomega(n)==3 \\ Charles R Greathouse IV, Apr 25 2016

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

Sequence in context: A115790 A025460 A169673 * A175854 A135133 A011712

Adjacent sequences: A101602 A101603 A101604 * A101606 A101607 A101608

easy,nonn

Jonathan Vos Post, Dec 09 2004

Description clarified by Antti Karttunen, Jul 23 2017

approved