A376579 - OEIS

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

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OFFSET

1,1

LINKS

Table of n, a(n) for n=1..82.

EXAMPLE

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

PROG

(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

CROSSREFS

Cf. A034693, A051686.

Sequence in context: A333253 A193990 A089367 * A130192 A175064 A349956

Adjacent sequences: A376576 A376577 A376578 * A376580 A376581 A376582

KEYWORD

sign

AUTHOR

Jean-Marc Rebert, Sep 29 2024

STATUS

approved