login

Year-end appeal: Please make a donation to the OEIS Foundation to support ongoing development and maintenance of the OEIS. We are now in our 61st year, we have over 378,000 sequences, and we’ve reached 11,000 citations (which often say “discovered thanks to the OEIS”).

A023505
Least odd prime divisor of prime(n) - 1, or 1 if prime(n) - 1 is a power of 2.
1
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
1,4
COMMENTS
Note that a(n)=1 for n= 1,2,3,7,55,6543, ... . - Michel Marcus, Oct 01 2013
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
Sequence in context: A162777 A241014 A173454 * A356489 A196997 A254934
KEYWORD
nonn
STATUS
approved