A053989 Smallest k such that nk-1 is prime.

3, 2, 1, 1, 4, 1, 2, 1, 2, 2, 4, 1, 8, 1, 2, 2, 4, 1, 2, 1, 2, 2, 6, 1, 6, 4, 2, 3, 6, 1, 2, 1, 4, 2, 4, 2, 2, 1, 6, 2, 4, 1, 6, 1, 2, 3, 6, 1, 2, 3, 2, 2, 4, 1, 2, 3, 2, 3, 6, 1, 8, 1, 4, 2, 6, 2, 6, 1, 2, 2, 4, 1, 14, 1, 2, 2, 4, 3, 2, 1, 8, 2, 4, 1, 6, 3, 2, 3, 16, 1, 2, 4, 6, 3, 4, 2, 2, 1, 2, 2

Offset: 1

Author

Henry Bottomley, Apr 04 2000

Keywords

• easy
• nonn

Examples

			a(5)=4 because the smallest prime in the sequence 5k-1 (4,9,14,19,24...) is 19 when k=4
		

Links

• Reinhard Zumkeller, Table of n, a(n) for n = 1..10000

Crossrefs

Cf. A034693, A038700, A071558, A010051, A103689, A200996.

Programs

Haskell

a053989 n = head [k | k <- [1..], a010051' (k * n - 1) == 1]
-- Reinhard Zumkeller, Feb 14 2013

PARI

a(n)=my(j);while(!isprime(j++*n-1),);j \\ Charles R Greathouse IV, Apr 18 2013

Formula

a(n) = (A038700(n)+1)/n.