5th Eötvös 1898 Problem 1 For which positive integers n does 3 divide 2 n + 1?
2 = 3-1, so 2 n + 1 = (-1) n + 1 mod 3. So it is divisible by 3 iff n odd. 5th Eötvös 1898 © John Scholes jscholes@kalva.demon.co.uk 29 Oct 2003 Last corrected/updated 29 Oct 03