=4a^2b^2-a^4-b^4-2a^2b^2
=-a^4-b^4+2a^2b^2
=-(a^2-b^2)^2
a^5b-8a^3b^3+16ab^5
=ab(a^4-8a^2b^2+16b^4)
=ab(a^2-4b^2)^2
=ab(a-2b)^2(a+2b)^2