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A101605 a(n) = 1 if n is a product of exactly 3 (not necessarily distinct) primes, otherwise 0. 26
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, 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, 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 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,1
LINKS
Eric Weisstein's World of Mathematics, Almost Prime.
FORMULA
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
EXAMPLE
a(28) = 1 because 28 = 2 * 2 * 7 is the product of exactly 3 primes, counted with multiplicity.
MAPLE
A101605 := proc(n)
if numtheory[bigomega](n) = 3 then
1;
else
0;
end if;
end proc: # R. J. Mathar, Mar 13 2015
MATHEMATICA
Table[Boole[PrimeOmega[n] == 3], {n, 100}] (* Jean-François Alcover, Mar 23 2020 *)
PROG
(PARI) is(n)=bigomega(n)==3 \\ Charles R Greathouse IV, Apr 25 2016
CROSSREFS
Cf. A010051, A064911, (char funct. of) A014612, A101637, A123074.
Sequence in context: A115790 A025460 A169673 * A358753 A175854 A353678
KEYWORD
easy,nonn
AUTHOR
Jonathan Vos Post, Dec 09 2004
EXTENSIONS
Description clarified by Antti Karttunen, Jul 23 2017
STATUS
approved

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Last modified March 19 01:34 EDT 2024. Contains 370952 sequences. (Running on oeis4.)