login
This site is supported by donations to The OEIS Foundation.

 

Logo

Please make a donation to keep the OEIS running. We are now in our 55th year. In the past year we added 12000 new sequences and reached 8000 citations (which often say "discovered thanks to the OEIS"). We need to raise money to hire someone to manage submissions, which would reduce the load on our editors and speed up editing.
Other ways to donate

Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A298207 Numbers that are a product of zero, one, or three (not necessarily distinct) prime numbers. 2
1, 2, 3, 5, 7, 8, 11, 12, 13, 17, 18, 19, 20, 23, 27, 28, 29, 30, 31, 37, 41, 42, 43, 44, 45, 47, 50, 52, 53, 59, 61, 63, 66, 67, 68, 70, 71, 73, 75, 76, 78, 79, 83, 89, 92, 97, 98, 99, 101, 102, 103, 105, 107, 109, 110, 113, 114, 116, 117, 124, 125, 127, 130 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

LINKS

Alois P. Heinz, Table of n, a(n) for n = 1..10000

FORMULA

Equals {A008578} union {A014612}.

Equals {A037144} minus {A001358}.

EXAMPLE

1 is a product of zero primes so is in the sequence.

6 = 2 * 3 is a product of two primes so is not in the sequence.

12 = 2 * 2 * 3 is a product of three primes so is in the sequence.

MAPLE

a:= proc(n) option remember; local k; for k

      from 1+`if`(n=1, 0, a(n-1)) while not

      numtheory[bigomega](k) in {0, 1, 3} do od; k

    end:

seq(a(n), n=1..70);  # Alois P. Heinz, Jan 15 2018

MATHEMATICA

Select[Range[200], MemberQ[{0, 1, 3}, PrimeOmega[#]]&]

PROG

(PARI) is(n) = my(v=[0, 1, 3]); #setintersect([bigomega(n)], v)==1 \\ Felix Fröhlich, Jan 15 2018

CROSSREFS

Cf. A000040, A001358, A001399, A008578, A014591, A014612, A026424, A028260, A298120, A298204, A298205.

Subsequence of A037144. - Felix Fröhlich, Jan 15 2018

Sequence in context: A166982 A026422 A026424 * A229125 A228853 A141832

Adjacent sequences:  A298204 A298205 A298206 * A298208 A298209 A298210

KEYWORD

nonn

AUTHOR

Gus Wiseman, Jan 14 2018

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 December 8 18:37 EST 2019. Contains 329865 sequences. (Running on oeis4.)