OFFSET
1,1
COMMENTS
All divisors of 4n-1 are odd.
LINKS
Nick Hobson, Table of n, a(n) for n = 1..1000
FORMULA
a(3k+1) = 3, k = 0,1,2,... a(5k+4) = 5 if k is not a multiple of 3; k = 1,2,4,5,7,8,... - Alexander Adamchuk, Nov 28 2006
a(n) = A020639(4n-1). - R. J. Mathar, Jan 23 2007
EXAMPLE
The prime divisors of 4*9 - 1 = 35 are 5 and 7, so a(9) = 5.
PROG
(PARI) vector(77, n, factor(4*n-1)[1, 1])
CROSSREFS
KEYWORD
easy,nonn
AUTHOR
Nick Hobson, Nov 26 2006
STATUS
approved