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A033993 Numbers that are divisible by exactly four different primes. 23

%I

%S 210,330,390,420,462,510,546,570,630,660,690,714,770,780,798,840,858,

%T 870,910,924,930,966,990,1020,1050,1092,1110,1122,1140,1155,1170,1190,

%U 1218,1230,1254,1260,1290,1302,1320,1326,1330,1365,1380,1386,1410,1428

%N Numbers that are divisible by exactly four different primes.

%C For a(n) < 30030 = 2 * 3 * 5 * 7 * 11 * 13 this is identical to "numbers with a semiprime number of distinct prime factors." - _Jonathan Vos Post_, Sep 21 2005

%H T. D. Noe, <a href="/A033993/b033993.txt">Table of n, a(n) for n = 1..1000</a>

%F a(n) has exactly 4 distinct prime factors. omega(a(n)) = A001221(a(n)) = 4. - _Jonathan Vos Post_, Sep 21 2005

%e The 4th primorial is the first term of this sequence: A002110(4) = 210.

%t Select[Range[1500], Length[FactorInteger[#]] == 4 &] (* _Vladimir Joseph Stephan Orlovsky_, Apr 22 2010 *)

%o (PARI) is(n)=omega(n)==4 \\ _Charles R Greathouse IV_, Sep 17 2015

%Y Cf. A000977, A007774, A000961, A002110, A033992, A051270, A001221.

%K nonn,easy

%O 1,1

%A _Labos Elemer_

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Last modified December 13 17:17 EST 2019. Contains 329970 sequences. (Running on oeis4.)