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[1] [2] Every positive integer is composite, prime, or the unit 1, so the composite numbers are exactly the numbers that are not prime and not a unit. [3] [4] E.g., the integer 14 is a composite number because it is the product of the two smaller integers 2 × 7 but the integers 2 and 3 are not because each can only be divided by one and itself ...
For example, 15 is a composite number because 15 = 3 · 5, but 7 is a prime number because it cannot be decomposed in this way. If one of the factors is composite, it can in turn be written as a product of smaller factors, for example 60 = 3 · 20 = 3 · (5 · 4).
In number theory, a Smith number is a composite number for which, in a given number base, the sum of its digits is equal to the sum of the digits in its prime factorization in the same base. In the case of numbers that are not square-free , the factorization is written without exponents, writing the repeated factor as many times as needed.
An alternative and equivalent definition of Carmichael numbers is given by Korselt's criterion.. Theorem (A. Korselt 1899): A positive composite integer is a Carmichael number if and only if is square-free, and for all prime divisors of , it is true that .
Some composite numbers (Carmichael numbers) have the property that a n − 1 is 1 (modulo n) for every a that is coprime to n. The smallest example is n = 561 = 3·11·17, for which a 560 is 1 (modulo 561) for all a coprime to 561.
The problem that we are trying to solve is: given an odd composite number, find its integer factors. To achieve this, Shor's algorithm consists of two parts: A classical reduction of the factoring problem to the problem of order-finding.
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For other numbers, the algorithm only returns “composite” with no further information. For example, consider n = 341 and a = 2. We have n − 1 = 85 × 4. Then 2 85 mod 341 = 32 and 32 2 mod 341 = 1. This tells us that n is a pseudoprime base 2, but not a strong pseudoprime base 2.