A295640 Smallest k not divisible by 5 such that k*5^n + 1 is prime.

2, 4, 2, 18, 12, 4, 12, 24, 26, 18, 42, 28, 2, 16, 18, 34, 92, 4, 12, 46, 26, 16, 6, 36, 26, 16, 6, 16, 152, 18, 42, 136, 6, 162, 132, 58, 24, 142, 48, 22, 56, 16, 36, 84, 2, 12, 24, 108, 168, 4, 12, 214, 36, 114, 12, 444, 26, 54, 416, 42

Offset: 1

Author

Pierre CAMI, Nov 25 2017

Keywords

• nonn

Comments

The ratio (Sum_(n=1..t) a(n)) / (Sum_(n=1..t) n) tends to log(5) as t increases.

Links

• Pierre CAMI, Table of n, a(n) for n = 1..2999

Crossrefs

Cf. A057778, A295639, A295641.

Programs

Mathematica

Array[Block[{k = 2}, While[Or[Divisible[k, 5], ! PrimeQ[k 5^# + 1]], k++]; k] &, 60] (* Michael De Vlieger, Dec 18 2017 *)

PARI

a(n) = {k = 1; while (!isprime(k*5^n+1), k++; if (!(k%5), k++)); k;} \\ Michel Marcus, Nov 25 2017