561

The number 561 is a composite number, meaning it has divisors other than 1 and itself. Specifically, 561 is known as a Carmichael number, which can sometimes mimic prime numbers because they satisfy certain mathematical properties that make them appear prime to some tests. Its prime factorization is 3 × 11 × 17, indicating that it can be divided evenly by these three prime numbers. Carmichael numbers are interesting in number theory because they challenge primality tests, highlighting the complexity of distinguishing between prime and composite numbers in mathematics.