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 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

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