Yun's algorithm

Question

I would like to try to implement Yun's algorithm for square-free factorization of polynomials. From Wikipedia (f is the polynomial):

a0 = gcd(f, f'); b1 = f/a0; c1 = f'/a0; d1 = c1 - b1'; i = 1
repeat
ai = gcd(bi, di); bi+1 = bi/ai; ci+1 = di/ai; i = i + 1; di = ci - bi'
until b = 1

However, I'm not sure about the second step. I would like to use it for polynomials with integer coefficients (not necessary monic or primitive). Is it possible to realize the division b1 = f/a0 using just integers?

I found the code for synthetic division:

def extended_synthetic_division(dividend, divisor):
    '''Fast polynomial division by using Extended Synthetic Division. Also works with non-monic polynomials.'''
    # dividend and divisor are both polynomials, which are here simply lists of coefficients. Eg: x^2 + 3x + 5 will be represented as [1, 3, 5]

    out = list(dividend) # Copy the dividend
    normalizer = divisor[0]
    for i in xrange(len(dividend)-(len(divisor)-1)):
        out[i] /= normalizer # for general polynomial division (when polynomials are non-monic),
                             # we need to normalize by dividing the coefficient with the divisor's first coefficient
        coef = out[i]
        if coef != 0: # useless to multiply if coef is 0
            for j in xrange(1, len(divisor)): # in synthetic division, we always skip the first coefficient of the divisor,
                                              # because it is only used to normalize the dividend coefficients
                out[i + j] += -divisor[j] * coef

    # The resulting out contains both the quotient and the remainder, the remainder being the size of the divisor (the remainder
    # has necessarily the same degree as the divisor since it is what we couldn't divide from the dividend), so we compute the index
    # where this separation is, and return the quotient and remainder.
    separator = -(len(divisor)-1)
    return out[:separator], out[separator:] # return quotient, remainder.

The problem for me is that out[i] /= normalizer. Would it always work with integer (floor) division for Yun's b1 = f/a0? Is it so that it is always possible to divide f/gcd(f, f')? Is the out[separator:] (remainder) always going to zero?

Answer 1

The fact that the "division in p/GCD(p, p') will always work (i.e. be "exact", with no remainder in Z)" follows from the definition of the GCD. For any polynomials p and q their GCD(p,q) divides both p and q exactly. That's why it is called GCD i.e. Greatest Common Divisor:

A greatest common divisor of p and q is a polynomial d that divides p and q and such that every common divisor of p and q also divides d.

P.S. it makes more sense to ask such purely mathematical questions at the more specialized https://math.stackexchange.com/