在质数集合中,“2”是惟一的偶数.正是“2”的这种特性,不难发现“2”在下列情况的存在性定理:(1)两个连续的自然数都是质数,则必有2(另一个是3)(2)两个质数的和与差是奇数,则必有2(3)两个质数的积是偶数,则必有2(4)两个质数的和与差仍为质数,则必有2“2”的存在性定理常常是在质数条件下解一些数学竞赛题的根据. In the prime number set, “2” is the only even number. It is this characteristic of “2” that it is not difficult to find the existence theorem of “2” in the following cases: (1) Two consecutive If the natural number is a prime number, then there must be 2 (the other is 3). (2) If the sum and difference of two prime numbers are odd, there must be 2 (3) products of two prime numbers that are even. There must be 2 (4) If the sum and difference of two prime numbers are still prime numbers, then the existence theorem of 2 “2 ” must be the basis for solving some mathematical contest problems under the conditions of prime numbers.