A023505 - OEIS

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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

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OFFSET

1,4

COMMENTS

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

LINKS

Table of n, a(n) for n=1..85.

FORMULA

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

PROG

(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

CROSSREFS