login
The OEIS Foundation is supported by donations from users of the OEIS and by a grant from the Simons Foundation.

 

Logo

Thanks to everyone who made a donation during our annual appeal!
To see the list of donors, or make a donation, see the OEIS Foundation home page.

Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A101605 a(n) = 1 if n is a product of exactly 3 (not necessarily distinct) primes, otherwise 0. 22
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

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

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-Fran├žois 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 * A175854 A135133 A011712

Adjacent sequences:  A101602 A101603 A101604 * A101606 A101607 A101608

KEYWORD

easy,nonn

AUTHOR

Jonathan Vos Post, Dec 09 2004

EXTENSIONS

Description clarified by Antti Karttunen, Jul 23 2017

STATUS

approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent
The OEIS Community | Maintained by The OEIS Foundation Inc.

License Agreements, Terms of Use, Privacy Policy. .

Last modified January 18 17:28 EST 2021. Contains 340254 sequences. (Running on oeis4.)