A playing card with a picture facing up, once turned, becomes a picture facing down. Turn it over again and its picture will go up again. If you keep flipping, you will find that when the number of flips is 2, 4, 6, 8 ..., the picture of playing cards is upward; When the flip number is 1, 3, 5, 7, 9…, the picture of playing cards is downward. In this way, "integers" are divided into two categories: one is 2, 4, 6, 8,10 ... called even numbers; The other is 1, 3, 5, 7, 9 ... called odd numbers.
In particular, 0 is an even number.
As can be clearly seen from the above, integers are divided into two categories, including
sequence
…,-2,- 1,0, 1,2,…
The numbers in are called integers. All the integers make up an integer set, which is a ring, marked Z (usually written as a hollow letter Z in modern times). The potential of ring z is Alef 0.
The given integer n can be negative (n∈Z-), nonnegative (n∈Z*), zero (n=0) or positive (n ∈ z+).
So let me tell you clearly, -2, -4 is an even number! (Please don't take it amiss.)