Composite number refers to the number divisible by other numbers (except 0) except 1 and itself in natural numbers.
Simply put, a number can only be divisible by 1 or itself to be a prime number, otherwise it is a composite number.
Example:
The smallest composite number is 4, which can be divisible by 1 and itself, or by 2.
The smallest prime number 2,2 can only be divisible by 1 and itself.
Calculation of prime numbers
1, there must be at least one prime number between a number greater than 1 and its twice (that is, within the interval (a, 2a)).
2. There is a prime arithmetic progression of any length.
3. An even number can be written as the sum of two composite numbers, and each composite number has at most 9 prime factors. (Norwegian mathematician Brown, 1920)
4. Even numbers must be written as prime numbers plus composite numbers, in which the number of factors of composite numbers has an upper bound. (Renee, 1948)
5. Even numbers must be written as a prime number plus a composite number consisting of at most five factors. Later, some people called this result (1+5) (Pan Chengdong, China, 1968).
6. A sufficiently large even number must be written as a prime number plus a composite number consisting of at most two prime factors. Short for (1+2)