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

1,1

Links

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

Formula

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

Example

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

Prog

(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

Crossrefs

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

Sequence in context: A143772 A373366 A373377 * A342459 A057160 A239938

Adjacent sequences: A053986 A053987 A053988 * A053990 A053991 A053992

Keyword

easy,nonn

Author

Henry Bottomley, Apr 04 2000

Status

approved