A023505 Least odd prime divisor of prime(n) - 1, or 1 if prime(n) - 1 is a power of 2.

1, 1, 1, 3, 5, 3, 1, 3, 11, 7, 3, 3, 5, 3, 23, 13, 29, 3, 3, 5, 3, 3, 41, 11, 3, 5, 3, 53, 3, 7, 3, 5, 17, 3, 37, 3, 3, 3, 83, 43, 89, 3, 5, 3, 7, 3, 3, 3, 113, 3, 29, 7, 3, 5, 1, 131, 67, 3, 3, 5, 3, 73, 3, 5, 3, 79, 3, 3, 173, 3, 11, 179, 3, 3, 3, 191, 97, 3, 5, 3, 11, 3, 5, 3, 3

Offset: 1

Author

Clark Kimberling

Keywords

• nonn

Comments

Note that a(n)=1 for n= 1,2,3,7,55,6543, ... . - Michel Marcus, Oct 01 2013

Crossrefs

Cf. A006093, A078701.

Programs

PARI

a(n) = my(p = prime(n) - 1, v = p/(2^valuation(p, 2))) ; if (v == 1, 1, factor(v)[1, 1]); \\ Michel Marcus, Oct 01 2013

Formula

a(n) = A078701(A006093(n)). - R. J. Mathar, Feb 06 2019