A255598 a(n) is the minimal number q>1 such that n(q+1)-1 is prime, or -1 if no such q exists.

2, 2, 3, 2, 3, 2, 5, 2, 5, 2, 3, 3, 7, 2, 3, 2, 3, 2, 5, 2, 3, 4, 5, 2, 5, 3, 3, 2, 5, 2, 13, 3, 3, 2, 3, 2, 11, 2, 5, 4, 3, 3, 5, 2, 3, 2, 5, 3, 5, 2, 9, 5, 3, 4, 7, 2, 3, 2, 5, 2, 7, 6, 3, 2, 5, 2, 5, 3, 11, 4, 3, 4, 13, 5, 5, 2, 3, 2, 7, 2, 7, 4, 3, 2, 5, 2, 3, 2, 15, 2, 7, 3, 5, 2, 3, 3, 11

Offset: 1

Author

Zak Seidov, Feb 27 2015

Keywords

• nonn

Comments

a(n) <= A060324(n), and a(n) = A060324(n) iff a(n) is prime.

Among first 10000 terms, in 2453 cases a(n) < A060324(n). First such cases are for n = 22, 40, 51, 54, 62, 70, 72, 82, 89, 100.

Also, among first 10^6 terms, in 324388 cases a(n) < A060324(n).

Crossrefs

Cf. A060324.

Programs

PARI

a(n) = {my(q = 2); while (! isprime(n*(q+1)-1), q ++); q;} \\ Michel Marcus, Feb 27 2015