2. Select Description in the View drop-down menu, pull the scroll bar, find the complement symbol, and click Insert.

3. Return to MathType editing interface, click MathType toolbar template, and select "superscript subscript template"-"subscript". Then enter a capital u in the subscript input box.

Extended data By definition, an empty set has 0 elements, or its potential is 0. However, the relationship between the two may go further: in the standard set theory definition of natural numbers, 0 is defined as an empty set. Real number 0 and empty set are two different concepts, so you can't sum 0 or {0}? Confused.

If a is a set, there is exactly one function f from {} to a, that is, an empty function. Therefore, empty set is the only initial object of set and function category.

Empty sets can only be transformed into topological spaces in one way, that is, by defining empty sets as open sets; This empty topological space is the only initial object of topological space category with continuous mapping.

An empty set is the proper subset of any non-empty set. ? There is only one subset, and there is no proper subset. {? } has two subsets, one is? One is itself.