f(x)没有重因式等价于f(x)与f'(x)没有公共根, 等价于f(x)与f'(x)互素.
一般都是用这个条件判别, 用辗转相除求最大公因式.
对f(x) = x^5-5x^4+7x^3-2x^2+4x-8,
f'(x) = 5x^4-20x^3+21x^2-4x+4.
5f(x) = (x-1)·f'(x)-6x^3+15x^2+12x-36 = (x-1)·f'(x)-3(2x^3-5x^2-4x+12).
设g(x) = 2x^3-5x^2-4x+12.
有4f'(x) = (10x-15)·g(x)+49x^2-196x+196 = (10x-15)·g(x)+49(x-2)^2.
设h(x) = (x-2)^2.
有g(x) = (2x+3)·h(x).
因此f(x)与f'(x)的最大公因式为(x-2)^2.
f(x)含有重因式(x-2)^3.