... of m and n On the other hand, suppose e is a divisor of m and n: e | m, n Then, working downwards, we find successively that e | m and e | n =⇒ e | r1 , e | r1 and e | m =⇒ e | r2 , e | r2 and ... exists a unique number d ∈ N such that d | m, d | n, and furthermore, if e ∈ N then e | m, e | n =⇒ e | d Definition 1.4 We call this number d the greatest common divisor of m and n, and we write ... bottom, d = rt | rt−1 , d | rt and d | rt−1 =⇒ d | rt−2 , d | rt−1 and d | rt−2 =⇒ d | rt−3 , d | r3 and d | r2 =⇒ d | r1 , d | r2 and d | r1 =⇒ d | m, d | r1 and d | m =⇒ d | n Thus d | m,...