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A033993
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Numbers that are divisible by exactly four different primes.
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18
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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; internal format)
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OFFSET
| 1,1
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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 (jvospost3(AT)gmail.com), Sep 21 2005
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LINKS
| T. D. Noe, Table of n, a(n) for n=1..1000
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FORMULA
| a(n) has exactly 4 distinct prime factors. omega(a(n)) = A001221(a(n)) = 4. - Jonathan Vos Post (jvospost3(AT)gmail.com), Sep 21 2005
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EXAMPLE
| The 4th primorial is the first: A002110[ 4 ]=210.
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MATHEMATICA
| Select[Range[0, 7! ], Length[FactorInteger[ # ]]==4&] [From Vladimir Orlovsky (4vladimir(AT)gmail.com), Apr 22 2010]
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CROSSREFS
| A000977, A007774, A000961, A002110, A033992, A051270.
Cf. A001221.
Sequence in context: A125011 A119427 A074159 * A046386 A046402 A147571
Adjacent sequences: A033990 A033991 A033992 * A033994 A033995 A033996
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KEYWORD
| nonn
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AUTHOR
| Labos E. (labos(AT)ana.sote.hu)
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