OFFSET
1,1
COMMENTS
4n-1 always has a prime divisor congruent to 3 modulo 4.
REFERENCES
T. M. Apostol, Introduction to Analytic Number Theory, Springer-Verlag, 1976, page 147.
LINKS
Nick Hobson, Table of n, a(n) for n = 1..1000
EXAMPLE
The divisors of 4*9 - 1 = 35 are 5 and 7, so a(9) = 7.
MATHEMATICA
Table[SelectFirst[Transpose[FactorInteger[4n-1]][[1]], Mod[#, 4]==3&], {n, 80}] (* The program uses the SelectFirst function from Mathematica version 10 *) (* Harvey P. Dale, Mar 05 2015 *)
PROG
(PARI) vector(76, n, f=factor(4*n-1); r=0; until(f[r, 1]%4==3, r++); f[r, 1])
CROSSREFS
KEYWORD
easy,nonn
AUTHOR
Nick Hobson, Nov 26 2006
STATUS
approved