

A037143


Numbers with at most 2 prime factors (counted with multiplicity).


33



1, 2, 3, 4, 5, 6, 7, 9, 10, 11, 13, 14, 15, 17, 19, 21, 22, 23, 25, 26, 29, 31, 33, 34, 35, 37, 38, 39, 41, 43, 46, 47, 49, 51, 53, 55, 57, 58, 59, 61, 62, 65, 67, 69, 71, 73, 74, 77, 79, 82, 83, 85, 86, 87, 89, 91, 93, 94, 95, 97, 101, 103, 106, 107, 109, 111, 113, 115, 118
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OFFSET

1,2


COMMENTS

A001222(a(n)) <= 2; A054576(a(n)) = 1.  Reinhard Zumkeller, Mar 10 2006
Products of two noncomposite numbers.  JuriStepan Gerasimov, Apr 15 2010
Also numbers such that exist permutations of all divisors only with coprime adjacent elements: A109810(a(n))>0.  Reinhard Zumkeller, May 24 2010
A060278(a(n)) = 0.  Reinhard Zumkeller, Apr 05 2013
1 together with numbers n such that sigma(n)+phi(n)d(n) = 2n2.  Wesley Ivan Hurt, May 03 2015


LINKS

R. Zumkeller, Table of n, a(n) for n = 1..1000


MAPLE

with(numtheory): A037143:=n>`if`(bigomega(n)<3, n, NULL): seq(A037143(n), n=1..200); # Wesley Ivan Hurt, May 03 2015


MATHEMATICA

Select[Range[120], PrimeOmega[#] <= 2 &] (* Ivan Neretin, Aug 16 2015 *)


PROG

(Haskell)
a037143 n = a037143_list !! (n1)
a037143_list = 1 : merge a000040_list a001358_list where
merge xs'@(x:xs) ys'@(y:ys) =
if x < y then x : merge xs ys' else y : merge xs' ys
 Reinhard Zumkeller, Dec 18 2012
(PARI) is(n)=bigomega(n)<3 \\ Charles R Greathouse IV, Apr 29 2015


CROSSREFS

Union of A008578 and A001358. Complement of A033942.
Cf. A063928, A001222, A054576, A109810, A060278.
A101040(a(n))=1 for n>1.
Subsequence of A037144.  Reinhard Zumkeller, May 24 2010
A098962 and A139690 are subsequences.
Sequence in context: A166155 A063538 A167207 * A236105 A292050 A048627
Adjacent sequences: A037140 A037141 A037142 * A037144 A037145 A037146


KEYWORD

nonn


AUTHOR

N. J. A. Sloane


EXTENSIONS

More terms from Henry Bottomley, Aug 15 2001


STATUS

approved



