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A119591 Least k such that 2*n^k - 1 is prime. 3
1, 1, 1, 4, 1, 1, 2, 1, 1, 2, 1, 2, 4, 1, 1, 2, 2, 1, 10, 1, 1, 6, 1, 2, 6, 1, 2, 136, 1, 1, 6, 6, 1, 6, 1, 1, 2, 2, 1, 2, 1, 2, 4, 1, 2, 4, 4, 1, 2, 1, 1, 44, 1, 1, 2, 1, 3, 2, 5, 3, 2, 2, 1, 4, 1, 768, 4, 1, 1, 52, 34, 2, 132, 1, 1, 14, 7, 1, 2, 2, 1, 8, 1, 2, 10, 1, 24, 60, 1, 1, 2, 3, 5, 2, 1, 1, 2, 1, 1 (list; graph; refs; listen; history; text; internal format)
OFFSET

2,4

COMMENTS

From Eric Chen, Jun 01 2015: (Start)

Conjecture: a(n) is defined for all n.

a(303) > 10000, a(304)..a(360) = {1, 2, 11, 1, 990, 1, 1, 2, 2, 4, 74, 5, 1, 10, 6, 6, 4, 1, 1, 2, 1, 9, 12, 1, 80, 2, 1, 1, 2, 14, 3, 2, 3, 1, 12, 1, 60, 36, 1, 8, 4, 34, 1, 522, 3, 15, 14, 1, 6, 2, 3, 1, 4, 5, 4, 10, 1}.

a(n) = 1 if and only if n is in A006254. (End)

LINKS

Eric Chen, Table of n, a(n) for n = 2..302

MATHEMATICA

f[n_] := Block[{k = 0}, While[ ! PrimeQ[2*n^k - 1], k++ ]; k ]; Table[f[n], {n, 2, 106}] (* Ray Chandler, Jun 08 2006 *)

PROG

(PARI) a(n) = for(k=1, 2^24, if(ispseudoprime(2*n^k-1), return(k))) \\ Eric Chen, Jun 01 2015

CROSSREFS

Cf. A119624, A253178.

Sequence in context: A183102 A178649 A335502 * A333782 A304876 A010125

Adjacent sequences:  A119588 A119589 A119590 * A119592 A119593 A119594

KEYWORD

nonn

AUTHOR

Pierre CAMI, Jun 01 2006

EXTENSIONS

Corrected and extended by Ray Chandler, Jun 08 2006

STATUS

approved

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Last modified March 1 03:32 EST 2021. Contains 341732 sequences. (Running on oeis4.)