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A114106
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Number of 4-almost primes less than or equal to 10^n.
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15
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0, 0, 12, 149, 1712, 18744, 198062, 2050696, 20959322, 212385942, 2139236881, 21454599814, 214499908019, 2139634739326, 21306682904040
(list; graph; refs; listen; history; internal format)
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OFFSET
| 0,3
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EXAMPLE
| There are 12 primes with four almost primes up to 100: 16,24,36,40,54,56,60,81,84,88,90&100.
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MATHEMATICA
| FourAlmostPrimePi[n_] := Sum[ PrimePi[n/(Prime@i*Prime@j*Prime@k)] - k + 1, {i, PrimePi[n^(1/4)]}, {j, i, PrimePi[(n/Prime@i)^(1/3)]}, {k, j, PrimePi@Sqrt[n/(Prime@i*Prime@j)]}]; Table[ FourAlmostPrimePi[n], {n, 0, 13}]
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CROSSREFS
| Cf. A006880, A066265, A109251, A014613.
Sequence in context: A167138 A001406 A057572 * A015610 A141326 A154733
Adjacent sequences: A114103 A114104 A114105 * A114107 A114108 A114109
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KEYWORD
| nonn
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AUTHOR
| Robert G. Wilson v (rgwv(AT)rgwv.com), Feb 07 2006
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EXTENSIONS
| a(14) from Robert G. Wilson v (rgwv(AT)rgwv.com), Jan 07 2007
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