A376579 Least prime p such that p*n+1 is prime, or -1 if no such prime exists.

2, 2, 2, 3, 2, 2, -1, 2, 2, 3, 2, 3, -1, 2, 2, 7, -1, 2, -1, 2, 2, 3, 2, 3, -1, 2, -1, 7, 2, 2, -1, 3, 2, 3, 2, 2, -1, 5, 2, 7, 2, 3, -1, 2, -1, 3, -1, 2, -1, 2, 2, 3, 2, 2, -1, 2, -1, 19, -1, 3, -1, 5, 2, 3, 2, 3, -1, 2, 2, 3, -1, 13, -1, 2, 2, 3, -1, 2, -1, 3, 2, 19

Offset: 1

Author

Jean-Marc Rebert, Sep 29 2024

Keywords

• sign

Examples

			a(1) = 2 because 2 is the smallest prime p such that 1*p+1 is prime.
		

Crossrefs

Cf. A034693, A051686.

Programs

PARI

a(n) = if (n%2, if (isprime(2*n+1), 2, -1), my(p=2); while(!isprime(p*n+1), p = nextprime(p+1)); p); \\ Michel Marcus, Sep 29 2024