xy(x²y²-1)-(x²y²-1)=(x²y²-1)(xy-1)
x3y3-x2y2-xy+1=x^2y^2(xy-1)-(xy-1)=(x^2y^2-1)(xy-1)=(xy+1)(xy-1)^2
x3y3-x2y2-xy+1 = x^2y^2(xy - 1) - (xy - 1) = (xy - 1)(x^2y^2 - 1) = (xy - 1)^2(x y + 1)
