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A126715
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a(n) is the smallest odd prime p such that p*2^n - 1 is prime.
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6
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3, 3, 3, 3, 3, 7, 3, 3, 5, 7, 5, 3, 5, 31, 5, 79, 17, 7, 3, 61, 17, 7, 83, 13, 83, 61, 11, 193, 83, 7, 521, 43, 5, 31, 3, 31, 17, 31, 3, 61, 107, 19, 53, 3, 557, 7, 23, 31, 5, 19, 11, 1033, 89, 307, 5, 3, 563, 79, 83, 733, 17, 79, 53, 61, 3, 67, 257, 43, 179, 139, 47, 73, 5, 421, 113
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OFFSET
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0,1
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COMMENTS
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By Xylouris' version of Linnik's theorem, a(n) << 2^(5.2n). - Charles R Greathouse IV, Dec 28 2011
a(n) = prime(k) for some k < 5*n, for the even prime 2*2^n-1 is prime for n = prime(k)-1. - Pierre CAMI, Jul 20 2014
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LINKS
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T. D. Noe and Pierre CAMI, Table of n, a(n) for n = 0..10000 (first 2501 terms from T. D. Noe)
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MATHEMATICA
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f[n_] := Block[{k = 2}, While[ !PrimeQ[ Prime[k]*2^n - 1], k++ ]; Prime@k]; Table[f@n, {n, 0, 74}] (* Robert G. Wilson v *)
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PROG
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(PARI) a(n) = p=3; t=2^n; while(!isprime(p*t-1), p=nextprime(p+1)); p \\ Colin Barker, Jul 22 2014
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CROSSREFS
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Sequence in context: A178154 A270774 A263144 * A158805 A163469 A105121
Adjacent sequences: A126712 A126713 A126714 * A126716 A126717 A126718
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KEYWORD
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nonn
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AUTHOR
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Pierre CAMI, Feb 13 2007
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EXTENSIONS
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More terms from Robert G. Wilson v, Feb 16 2007
Entries checked by N. J. A. Sloane, Mar 02 2007 and some errors corrected.
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STATUS
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approved
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