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 A014613 Numbers that are products of 4 primes (these numbers are sometimes called "4-almost primes", a generalization of semiprimes). 135
 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 a(n) = A078840(4,n). - R. J. Mathar, Jan 30 2019 MATHEMATICA Select[Range, Plus @@ Last /@ FactorInteger[ # ] == 4 &] (* Vladimir Joseph Stephan Orlovsky, Apr 23 2008 *) Select[Range, PrimeOmega[#] == 4&] (* Jean-François Alcover, Jan 17 2014 *) PROG (PARI) isA014613(n) = bigomega(n)==4 \\ 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 * A323350 A307341 A046370 Adjacent sequences:  A014610 A014611 A014612 * A014614 A014615 A014616 KEYWORD nonn AUTHOR EXTENSIONS More terms from Patrick De Geest, Jun 15 1998 STATUS approved

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Last modified November 13 10:50 EST 2019. Contains 329093 sequences. (Running on oeis4.)