The characteristics of a set should be clear: certainty (not ambiguity, but existence, which must be clear, yes, no, no); Reciprocity (elements in a set cannot be repeated, even if they are different in form and identical in substance); Disorder (elements in a set can change their order at will, but the set belongs to the same set)

In addition, it is necessary to distinguish between an empty set (there are no elements in the set, not even 0) and {0} (there are elements in this set, which is 0);

Finally, we should be clear about three operations of the set:

Cross, take a set composed of common elements in two sets;

And, take all the elements in the two sets and remove the duplicate elements to form a set.

The complement set of A is a set, which consists of all the elements removed from the complete set U in set A and the remaining elements.

If element a is an element in the intersection of sets a and b, then a is an element in sets a and b;

If element A is an element in the union of sets A and B, it may be an element in set A, an element in set B, or an element contained in both sets A and B..

If element a is the complement of set a, then element a must not be in set a, but in a complete set;