3*x**3 + c4*x**4 + x**5, x)
        >>> Matrix.companion(p)
        Matrix([
        [0, 0, 0, 0, -c0],
        [1, 0, 0, 0, -c1],
        [0, 1, 0, 0, -c2],
        [0, 0, 1, 0, -c3],
        [0, 0, 0, 1, -c4]])
        z