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A014613 Numbers that are products of 4 primes (these numbers are sometimes called "4-almost primes", a generalization of semiprimes). 121
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 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Also called "quadruprimes". Divisible by exactly 4 prime powers (not including 1).

LINKS

T. D. Noe, Table of n, a(n) for n = 1..10000

Eric Weisstein's World of Mathematics, Almost Prime.

FORMULA

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

MATHEMATICA

Select[Range[200], Plus @@ Last /@ FactorInteger[ # ] == 4 &] (* Vladimir Orlovsky, Apr 23 2008 *)

Select[Range[400], PrimeOmega[#] == 4&] (* Jean-Fran├žois Alcover, Jan 17 2014 *)

PROG

(PARI) isA014613(n) = bigomega(n)==4 [From Michael B. Porter, Dec 13 2009]

CROSSREFS

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

KEYWORD

nonn

AUTHOR

Eric W. Weisstein

EXTENSIONS

More terms from Patrick De Geest, Jun 15 1998.

STATUS

approved

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Last modified April 23 14:29 EDT 2014. Contains 240931 sequences.