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Second, judgment.

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Third, choose

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This part of the extended information mainly examines the knowledge points of composite numbers and prime numbers:

One of the integers greater than 1, which is divisible by other numbers (except 0) except 1 and itself. In contrast, it is a prime number, and 1 is neither a prime number nor a composite number. The smallest composite number is 4. Among them, the perfect number and the number of blind dates are based on this.

All even numbers greater than 2 are composite numbers, and all odd numbers greater than 5 are composite numbers. Except 0, all natural numbers with 0 bits are composite numbers. All natural numbers with units of 4, 6 and 8 are composite numbers. The minimum (even) composite number is 4 and the minimum odd composite number is 9. Every composite number can be written as the unique form of the product of prime numbers, that is, the factorization of prime factors.

Among natural numbers greater than 1, the prime number has no other factors except 1 and itself. The number of prime numbers is infinite. There is a classic proof in Euclid's Elements of Geometry. It uses a common proof method: reduction to absurdity. The concrete proof is as follows: Suppose there are only a limited number of prime numbers, which are arranged in sequence from small to large as p 1, p2, pn, and let n = p1× p2××××× pn, then it is a prime number.