A193807 Smallest positive integer k such that n*k^2 + 1 is a prime.

1, 1, 2, 1, 6, 1, 2, 3, 2, 1, 6, 1, 2, 3, 2, 1, 6, 1, 8, 3, 4, 1, 6, 2, 2, 6, 2, 1, 12, 1, 6, 6, 8, 2, 24, 1, 2, 9, 2, 1, 30, 1, 2, 3, 2, 1, 6, 2, 2, 6, 10, 1, 24, 3, 4, 6, 2, 1, 30, 1, 4, 9, 4, 2, 6, 1, 2, 3, 2, 1, 6, 1, 2, 12, 4, 4, 36, 1, 2, 9, 4, 1, 12

Offset: 1

Author

Michel Lagneau, Aug 05 2011

Keywords

• nonn

Examples

			a(5) = 6 because 5*6^2 + 1 = 181 is prime.
		

Links

• Amiram Eldar, Table of n, a(n) for n = 1..10000

Programs

Mathematica

Table[k = 0; While[!PrimeQ[n*k^2 + 1], k++]; k, {n, 110}]

PARI

a(n) = my(k=1); while (!isprime(n*k^2 + 1), k++); k; \\ Michel Marcus, Aug 19 2019