(a-1)(a+1)(a^2+1)(a^4+1)(a^8+1)-a^16
=(a^2-1)(a^2+1)(a^4+1)(a^8+1)-a^16
=(a^4-1)(a^4+1)(a^8+1)-a^16
=(a^8-1)(a^8+1)-a^16
=a^16-1-a^16
=-1
(a-1)(a+1)(a^2+1)(a^4+1)(a^8+1)-a^16
=(a^2-1)(a^2+1)(a^4+1)(a^8+1)-a^16
=(a^4-1)(a^4+1)(a^8+1)-a^16
=(a^8-1)(a^8+1)-a^16
=a^16-1-a^16
=-1