A255707 Least number k > 0 such that (2*n-1)^k - 2 is prime, or 0 if no such number exists.

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

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: A299203 A329036 A205552 * A260757 A157114 A156786

Adjacent sequences: A255704 A255705 A255706 * A255708 A255709 A255710

Keyword

nonn

Author

Robert Price, Mar 02 2015

Status

approved