Example 2.10

Example 2.10: Let $$X = \{1,2,3\}$$, and let $$T = \{\emptyset, X, \{1,2\}, \{2,3\}, \{2\}\}$$. Then because $$\{2\}$$ is an open set, its complement in $$X$$, $$\{1,3\}$$, is a closed set.