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A033993 Numbers that are divisible by exactly four different primes. 19
210, 330, 390, 420, 462, 510, 546, 570, 630, 660, 690, 714, 770, 780, 798, 840, 858, 870, 910, 924, 930, 966, 990, 1020, 1050, 1092, 1110, 1122, 1140, 1155, 1170, 1190, 1218, 1230, 1254, 1260, 1290, 1302, 1320, 1326, 1330, 1365, 1380, 1386, 1410, 1428 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

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

LINKS

T. D. Noe, Table of n, a(n) for n = 1..1000

FORMULA

a(n) has exactly 4 distinct prime factors. omega(a(n)) = A001221(a(n)) = 4. - Jonathan Vos Post, Sep 21 2005

EXAMPLE

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

MATHEMATICA

Select[Range[1500], Length[FactorInteger[#]] == 4 &] (* Vladimir Joseph Stephan Orlovsky, Apr 22 2010 *)

PROG

(PARI) is(n)=omega(n)==4 \\ Charles R Greathouse IV, Sep 17 2015

CROSSREFS

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

Sequence in context: A119427 A127424 A074159 * A046386 A229272 A046402

Adjacent sequences:  A033990 A033991 A033992 * A033994 A033995 A033996

KEYWORD

nonn,easy

AUTHOR

Labos Elemer

STATUS

approved

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Last modified July 21 20:44 EDT 2017. Contains 289648 sequences.