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A059788
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a(n) = largest prime < 2*prime(n).
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13
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3, 5, 7, 13, 19, 23, 31, 37, 43, 53, 61, 73, 79, 83, 89, 103, 113, 113, 131, 139, 139, 157, 163, 173, 193, 199, 199, 211, 211, 223, 251, 257, 271, 277, 293, 293, 313, 317, 331, 337, 353, 359, 379, 383, 389, 397, 421, 443, 449, 457, 463, 467, 479, 499, 509, 523
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OFFSET
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1,1
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COMMENTS
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Also, smallest member of the first pair of consecutive primes such that between them is a composite number divisible by the n-th prime. - Amarnath Murthy, Sep 25 2002
Except for its initial term, A006992 is a subsequence based on iteration of n -> A151799(2n). The range of this sequence is a subset of A065091. - M. F. Hasler, May 08 2016
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LINKS
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FORMULA
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EXAMPLE
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n=18: p(18)=61, so a(18) is the largest prime below 2*61=122, which is 113.
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MAPLE
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with(numtheory):
prevprime(2*ithprime(n)) ;
end proc:
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MATHEMATICA
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a[n_] := Prime[PrimePi[2Prime[n]]]
NextPrime[2*Prime[Range[100]], -1] (* Zak Seidov, May 08 2016 *)
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PROG
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(PARI) a(n) = precprime(2*prime(n)); \\ Michel Marcus, May 08 2016
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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