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Common divisors
The greatest common divisor of two numbers is the largest number that divides both of them with no remainder.
For 12 and 18: divisors of 12 are 1, 2, 3, 4, 6, 12. Divisors of 18 are 1, 2, 3, 6, 9, 18. Common divisors are 1, 2, 3, 6. The greatest is 6.
We write gcd(12, 18) = 6. This number measures how much structure the two numbers share.
