A023519 Least odd prime divisor of prime(n)*prime(n-1) - 1, or 1 if prime(n)*prime(n-1) - 1 is a power of 2.

1, 5, 7, 17, 19, 71, 5, 7, 109, 3, 449, 3, 379, 881, 5, 3, 3, 7, 3, 29, 2591, 3, 11, 3, 13, 31, 7, 5, 7, 3079, 5, 4159, 3, 9521, 5, 7, 3, 3, 5, 3, 3, 97, 5, 7, 5, 17, 3, 3, 5, 7, 13339, 3, 31, 5, 3, 3, 3, 7, 3, 11, 39761, 11, 5, 23869, 7, 5, 23, 3, 59, 151, 19, 3, 43, 3, 3, 11, 3, 19, 39799, 13

Offset: 1

Author

Clark Kimberling

Keywords

• nonn

Comments

Assumes the not generally accepted convention prime(0) = 1. - Michel Marcus, Jun 06 2019

Crossrefs

Cf. A023515, A078701.

Programs

PARI

p(n) = if (n==0, 1, prime(n));
f(n) = my(v = select(x->((x%2)==1), factor(n)[, 1])); if (#v, vecmin(v), 1);
a(n) = f(p(n)*p(n-1) - 1); \\ Michel Marcus, Jun 06 2019

Formula

a(n) = A078701(A023515(n)). - Michel Marcus, Jun 06 2019

Extensions

Terms corrected by Sean A. Irvine, Jun 05 2019