Enclose several numbers in braces, and separate two adjacent numbers with commas. For example, {1, 2} and {1, 3,5} are called sets, and each number is called an element of this set. In a set, the rational number X is an element of it. If (user-defined number) -x,

{1, 3,5} is a collection of gold; Because 6- 1=5, and 5 is an element of the set 1, 3,5}; 6-3 = 3,3 is also an element of the set {1, 3,5}; 6-5= 1, and 1 is also an element of the set {1, 3,5}, so {1, 3,5} is a golden set; Write two sets of gold, such as {0,6} and {2,3,4}.

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When some specified objects are gathered together, they become a set, which contains finite elements and infinite elements. An empty set is a set without any elements, and it is recorded as φ. An empty set is a subset of any set, the proper subset of any non-empty set, any set is a subset of itself, and the subset and proper subset are transitive.

If all elements of set A are elements of set B at the same time, then A is called a subset of B. If A is a subset of B and A is not equal to B, then A is called the proper subset of B. Write A? B. Everyone's collection is everyone's collection, proper subset.

Reference source: Baidu Encyclopedia-Mathematics Highlights