a^4b-16b+a^4-16=a^4(b+1)-16(b+1)=(a^4-16)(b+1)=(a^2+1)(a^2-1)(b+1)=
=(a^2+1)(a+1)(a-1)(b+1)
=(a^4b-16b)+(a^4-16)=b(a^4-16)+(a^4-16)=(b+1)(a^4-16)=(b+1)(a^2-4)(a^2+4)=(b+1)(a-2)(a+2)(a^2+4).
