因数，倍数，余数，不完全商
b | a, c | b => c | a
a | m. b | m => (a + b) | m
a | m, q∈Z => aq | m
公因数，最大公因数，互质（素），两两互质
两两互质 => 互质 <=> 最大公因数为1
(a, b) = (|a|, |b|)
(0, b) = |b|
(a, b) = (b, a mod b)
辗转相除法
(a, b) = (b, r1) = (r1, r2) = ... = (rn, 0) = |rn|, r1 ≡ a mod b, ……
(am, bm) = (a, b)m
(a, b) = c, (c, d) = e => (a, b, c) = e
最小公倍数
a * b = (a, b) * [a, b]