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A255707 Least number k > 0 such that (2*n-1)^k - 2 is prime, or 0 if no such number exists. 3
0, 2, 1, 1, 1, 4, 1, 1, 6, 1, 1, 24, 1, 2, 2, 1, 1, 2, 2, 1, 4, 1, 1, 2, 1, 8, 4, 1, 12, 4, 1, 1, 8, 3, 1, 2, 1, 1, 2, 38, 1, 4, 1, 4, 2, 1, 2, 4, 747, 1, 4, 1, 1, 2, 1, 1, 10, 1, 2, 2, 2, 6, 42, 2, 1, 2, 1, 2, 10, 1, 1, 4, 2, 16, 50, 1, 1, 2, 22, 1, 2, 38 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

LINKS

Michel Marcus, Table of n, a(n) for n = 1..152 (terms 1..143 from Robert Price)

Carlos Rivera, Puzzle 887. p(n)^c-2 is prime, The Prime Puzzles and Problems Connection.

FORMULA

a(A098090(n)) = 1. - Michel Marcus, Mar 03 2015

MATHEMATICA

lst = {0}; For[n = 2, n ≤ 143, n++, For[k = 1, k >= 1, k++, If[PrimeQ[(2*n - 1)^k - 2], AppendTo[lst, k]; Break[]]]]; lst

PROG

(PARI) a(n)=if(n==1, return(0)); k=1; while(k, if(ispseudoprime((2*n-1)^k-2), return(k)); k++)

vector(50, n, a(n)) \\ Derek Orr, Mar 03 2015

CROSSREFS

Cf. A079706, A084712, A084713, A084714, A098090, A138066.

Sequence in context: A010121 A174726 A205552 * A260757 A157114 A156786

Adjacent sequences:  A255704 A255705 A255706 * A255708 A255709 A255710

KEYWORD

nonn

AUTHOR

Robert Price, Mar 02 2015

STATUS

approved

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Last modified August 19 08:57 EDT 2017. Contains 290794 sequences.