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A101446
a(n)= least k such that n^k*(n^k-1)-1 is prime with n > 1.
1
2, 1, 1, 1, 1, 1, 3, 1, 1, 1, 1, 10, 1, 5, 1, 1, 2, 2, 1, 1, 1, 2, 4, 1, 4, 1, 12, 1, 328, 1, 1, 6, 24, 77, 1, 8, 75, 1, 1, 2, 1, 2, 13, 1, 1, 1, 14, 1, 3, 1, 45, 3, 1, 1, 1, 1, 8, 625, 1, 1, 22, 3, 3, 1, 1, 1, 11, 1, 4, 1, 2
OFFSET
2,1
COMMENTS
a(73)>2475. - Ray Chandler, Jan 27 2005
EXAMPLE
2^2*(2^2-1)-1=11 prime so a(2)=2.
MATHEMATICA
f[n_] := Block[{k = 1, c}, While[c = n^k; ! PrimeQ[c*(c - 1) - 1], k++ ]; Return[k]; ]; Table[f[n], {n, 2, 72}] (* Ray Chandler, Jan 27 2005 *)
PROG
(PARI) a(n) = {my(k=1); while (!ispseudoprime(n^k*(n^k-1)-1), k++); k; } \\ Michel Marcus, Sep 16 2019
CROSSREFS
Cf. A101406.
Sequence in context: A371213 A323719 A220122 * A333769 A259396 A328672
KEYWORD
nonn,more
AUTHOR
Pierre CAMI, Jan 24 2005
EXTENSIONS
a(55)-a(72) from Ray Chandler, Jan 27 2005
STATUS
approved