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

1,2

Comments

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

Links

Table of n, a(n) for n=1..80.

Formula

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

Prog

(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

Crossrefs

Cf. A023515, A078701.

Sequence in context: A168245 A242929 A128490 * A023517 A128491 A146949

Adjacent sequences: A023516 A023517 A023518 * A023520 A023521 A023522

Keyword

nonn

Author

Clark Kimberling

Extensions

Terms corrected by Sean A. Irvine, Jun 05 2019

Status

approved