Let a, b be integers. d is called*a* gcd of a and b when it has the following two properties: (a) d|a (read: d divides a or a in a multiple of d or the equation dx = a has an integer solution x) and d|b. {Thus, d is a common divisor of a and b.} (b) If d'|a and d'|b, then d'|d. {Thus, d is a multiple of every other common divisor of a and b and is the 'greatest' in the sense that a divisor is 'sma