1/7 repeats with period 6 · period = order of 10 mod 7 · Fermat little theorem · runs locally
period of 1/n = smallest k such that 10^k ≡ 1 (mod n) = order of 10 in (ℤ/nℤ)* · period divides φ(n) by Euler's theorem · for prime p, φ(p)=p−1; 1/7 has period 6=φ(7) (10 is a primitive root mod 7)