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A020900
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Greatest k such that k-th prime < twice n-th prime.
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14
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2, 3, 4, 6, 8, 9, 11, 12, 14, 16, 18, 21, 22, 23, 24, 27, 30, 30, 32, 34, 34, 37, 38, 40, 44, 46, 46, 47, 47, 48, 54, 55, 58, 59, 62, 62, 65, 66, 67, 68, 71, 72, 75, 76, 77, 78, 82, 86, 87, 88, 90, 91, 92, 95, 97, 99, 99, 100, 101, 102, 103, 106, 112
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history;
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OFFSET
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1,1
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LINKS
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Harvey P. Dale, Table of n, a(n) for n = 1..1000
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FORMULA
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a(n) = A000720(A100484(n)). - Michel Marcus, Feb 02 2020
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EXAMPLE
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4th prime is 7, twice the 4th prime is 14, the greatest prime < 14 is 13 that is the 6th prime, hence, a(4) = 6. - Bernard Schott, Feb 02 2020
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MATHEMATICA
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PrimePi[NextPrime[#, -1]]&/@(2Prime[Range[70]]) (* Harvey P. Dale, Jul 05 2012 *)
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PROG
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(PARI) a(n) = primepi(2*prime(n)); \\ Michel Marcus, Oct 25 2017; Feb 02 2020
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CROSSREFS
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Cf. A000720 (pi(n)), A100484 (2*prime(n)).
Cf. A020901, A059788, A060265.
Cf. A102820 (first differences).
Sequence in context: A233133 A186541 A207432 * A333309 A282697 A002479
Adjacent sequences: A020897 A020898 A020899 * A020901 A020902 A020903
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KEYWORD
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nonn
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AUTHOR
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Clark Kimberling
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STATUS
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approved
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