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A014613
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Numbers that are products of 4 primes (these numbers are sometimes called "4-almost primes", a generalization of semiprimes).
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120
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16, 24, 36, 40, 54, 56, 60, 81, 84, 88, 90, 100, 104, 126, 132, 135, 136, 140, 150, 152, 156, 184, 189, 196, 198, 204, 210, 220, 225, 228, 232, 234, 248, 250, 260, 276, 294, 296, 297, 306, 308, 315, 328, 330, 340, 342, 344, 348, 350, 351, 364, 372, 375, 376
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OFFSET
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1,1
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COMMENTS
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Also called "quadruprimes". Divisible by exactly 4 prime powers (not including 1).
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LINKS
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T. D. Noe, Table of n, a(n) for n = 1..10000
Eric Weisstein's World of Mathematics, Almost Prime.
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FORMULA
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Product p_i^e_i with Sum e_i = 4.
a(n) ~ 6n log n / (log log n)^3. - Charles R Greathouse IV, May 04 2013
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MATHEMATICA
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Select[Range[200], Plus @@ Last /@ FactorInteger[ # ] == 4 &] (* Vladimir Orlovsky, Apr 23 2008 *)
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PROG
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(PARI) isA014613(n) = bigomega(n)==4 [From Michael B. Porter, Dec 13 2009]
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CROSSREFS
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Cf. A033987, A114106 (number of 4-almost primes <= 10^n).
Sequences listing r-almost primes; that is the n such that A001222(n) = r: A000040 (r = 1), A001358 (r = 2), A014612 (r = 3), this sequence (r = 4), A014614 (r = 5), A046306 (r = 6), A046308 (r = 7), A046310 (r = 8), A046312 (r = 9), A046314 (r = 10), A069272 (r = 11), A069273 (r = 12), A069274 (r = 13), A069275 (r = 14), A069276 (r = 15), A069277 (r = 16), A069278 (r = 17), A069279 (r = 18), A069280 (r = 19), A069281 (r = 20). - Jason Kimberley, Oct 02 2011
Sequence in context: A036328 A067028 A110893 * A046370 A103248 A140135
Adjacent sequences: A014610 A014611 A014612 * A014614 A014615 A014616
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KEYWORD
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nonn
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AUTHOR
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Eric W. Weisstein
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EXTENSIONS
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More terms from Patrick De Geest, Jun 15 1998.
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STATUS
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approved
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