The original formula = (2-1) (2+1) (2 2+1) (2 4+1) (2 8+1).
=(2^2- 1)(2^2+ 1)(2^4+ 1)(2^8+ 1)
=(2^4- 1)(2^4+ 1)(2^8+ 1)
=(2^8- 1)(2^8+ 1)
=2^ 16- 1