A075818 Even numbers with exactly 3 prime factors (counted with multiplicity).

8, 12, 18, 20, 28, 30, 42, 44, 50, 52, 66, 68, 70, 76, 78, 92, 98, 102, 110, 114, 116, 124, 130, 138, 148, 154, 164, 170, 172, 174, 182, 186, 188, 190, 212, 222, 230, 236, 238, 242, 244, 246, 258, 266, 268, 282, 284, 286, 290, 292, 310, 316, 318, 322, 332, 338

Offset

1,1

Comments

Twice the semiprime numbers. - Juri-Stepan Gerasimov, Jun 01 2010

Links

Vincenzo Librandi, Table of n, a(n) for n = 1..2626

Formula

a(n)=2*A001358(n). - Juri-Stepan Gerasimov, Jun 01 2010

Example

28=2^2*7, 30=2*3*5 and 42=2*3*7 are even and are products of exactly 3 primes.

Maple

ts_bo3_sod := proc(n); if (numtheory[bigomega](n)=3 and type(n, even)='true') then RETURN(n); fi end: abo3sod := [seq(ts_bo3_sod(i), i=1..2300)]: abo3sod;

Mathematica

Select[Range[100], Plus@@Last/@FactorInteger[#]==2&] 2 (* Vincenzo Librandi, Nov 10 2018 *)
Select[Range[2, 400, 2], PrimeOmega[#]==3&] (* Harvey P. Dale, Oct 15 2021 *)

Prog

(PARI) list(lim)=my(v=List()); forprime(p=2, lim\4, forprime(q=2, min(lim\p\2, p), listput(v, 2*p*q))); Set(v) \\ Charles R Greathouse IV, Aug 29 2017
(Magma) [2*n: n in [2..200] | &+[d[2]: d in Factorization(n)] eq 2]; // Vincenzo Librandi Nov 10 2018

Crossrefs

Cf. A046470, A053858.

Sequence in context: A370650 A285508 A054397 * A090738 A085103 A280968

Adjacent sequences: A075815 A075816 A075817 * A075819 A075820 A075821

Keyword

easy,nonn

Author

Jani Melik, Oct 13 2002

Extensions

Edited by Dean Hickerson, Oct 21 2002

Status

approved