A078178 Least k>=2 such that n^k + n - 1 is prime.

2, 2, 2, 2, 2, 3, 2, 2, 2, 2, 4, 2, 16, 2, 2, 4, 3, 2, 2, 2, 7, 4, 2, 3, 2, 3, 2, 10, 2, 2, 108, 3, 6, 2, 3, 7, 2, 2, 4, 2, 16, 3, 2, 2, 2, 20, 2, 7, 2, 3, 3, 2, 2, 2, 2, 9, 4, 2, 2, 7, 8, 3, 2, 2, 2, 24, 2, 6, 2, 12, 4, 3, 8, 6, 2, 4, 3, 9, 194, 3, 13, 2, 8, 2, 2, 3, 8, 2, 10, 6, 4, 2, 2, 54, 2, 132, 4, 10, 2

Offset: 2

Author

Reinhard Zumkeller, Nov 20 2002

Keywords

• nonn

Comments

n^a(n) + n - 1 = A078179(n).

Examples

			7^2+7-1=5*11, but 7^3+7-1=349=A000040(70), therefore a(7)=3.
		

Crossrefs

Cf. A076845, A078179.

Cf. A010051.

Programs

Haskell

a078178 n = head [k | k <- [2..], a010051'' (n ^ k + n - 1) == 1]
-- Reinhard Zumkeller, Jul 16 2014

Mathematica

lkp[n_]:=Module[{k=2},While[!PrimeQ[n^k+n-1],k++];k]; Array[lkp,100,2] (* Harvey P. Dale, May 24 2020 *)

Extensions

More terms from Benoit Cloitre, Nov 20 2002