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The greatest common divisor (GCD) of integers a and b, at least one of which is nonzero, is the greatest positive integer d such that d is a divisor of both a and b; that is, there are integers e and f such that a = de and b = df, and d is the largest such integer.
d() is the number of positive divisors of n, including 1 and n itself; σ() is the sum of the positive divisors of n, including 1 and n itselfs() is the sum of the proper divisors of n, including 1 but not n itself; that is, s(n) = σ(n) − n
A least common multiple of a and b is a common multiple that is minimal, in the sense that for any other common multiple n of a and b, m divides n. In general, two elements in a commutative ring can have no least common multiple or more than one. However, any two least common multiples of the same pair of elements are associates. [10]
7 is a divisor of 42 because =, so we can say It can also be said that 42 is divisible by 7, 42 is a multiple of 7, 7 divides 42, or 7 is a factor of 42. The non-trivial divisors of 6 are 2, −2, 3, −3.
One way to classify composite numbers is by counting the number of prime factors. A composite number with two prime factors is a semiprime or 2-almost prime (the factors need not be distinct, hence squares of primes are included).
637: 63 + 7 × 4 = 91, 9 + 1 × 4 = 13. Subtract the last two digits from four times the rest. The result must be divisible by 13. 923: 9 × 4 − 23 = 13. Subtract 9 times the last digit from the rest. The result must be divisible by 13. (Works because 91 is divisible by 13). 637: 63 − 7 × 9 = 0. 14: It is divisible by 2 and by 7. [6]
m and n are coprime (also called relatively prime) if gcd(m, n) = 1 (meaning they have no common prime factor). lcm(m, n) (least common multiple of m and n) is the product of all prime factors of m or n (with the largest multiplicity for m or n). gcd(m, n) × lcm(m, n) = m × n. Finding the prime factors is often harder than computing gcd and ...
63 is a Mersenne number of the form with an of , [5] however this does not yield a Mersenne prime, as 63 is the forty-fourth composite number. [6] It is the only number in the Mersenne sequence whose prime factors are each factors of at least one previous element of the sequence ( 3 and 7 , respectively the first and second Mersenne primes). [ 7 ]