Theorem 8.9. Let f : X Y be a continuous, surjective function. If X is connected, then Y is connected.

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**Theorem 8.9.** Let \( f : X \rightarrow Y \) be a continuous, surjective function. If \( X \) is connected, then \( Y \) is connected.

**Definition.** Let \( X \) be a topological space. Then \( X \) is **connected** if and only if \( X \) is not the union of two disjoint non-empty open sets.

**Definition.** Let \( X \) be a topological space. Subsets \( A, B \) in \( X \) are **separated** if and only if \( \overline{A} \cap B = A \cap \overline{B} = \emptyset \). Thus \( B \) does not contain any limit points of \( A \), and \( A \) does not contain any limit points of \( B \). The notation \( X = A \mid B \) means \( X = A \cup B \) and \( A \) and \( B \) are separated sets.

**Theorem 8.1.** The following are equivalent:

1. \( X \) is connected.
2. There is no continuous function \( f : X \rightarrow \mathbb{R}_{\text{std}} \) such that \( f(X) = \{0, 1\} \).
3. \( X \) is not the union of two disjoint non-empty separated sets.
4. \( X \) is not the union of two disjoint non-empty closed sets.
5. The only subsets of \( X \) that are both closed and open in \( X \) are the empty set and \( X \) itself.
6. For every pair of points \( p \) and \( q \) and every open cover \(\{U_{\alpha}\}_{\alpha \in \lambda}\) of \( X \) there exist a finite number of the \( U_{\alpha} \)'s, \(\{U_{\alpha_1}, U_{\alpha_2}, U_{\alpha_3}, \ldots, U_{\alpha_n}\}\) such that \( p \in U_{\alpha_1} \), \( q \in U_{\alpha_n} \), and for each \( i < n \), \( U_{\alpha_i} \cap U_{\alpha_{i+1}} \neq \emptyset \).
Transcribed Image Text:**Theorem 8.9.** Let \( f : X \rightarrow Y \) be a continuous, surjective function. If \( X \) is connected, then \( Y \) is connected. **Definition.** Let \( X \) be a topological space. Then \( X \) is **connected** if and only if \( X \) is not the union of two disjoint non-empty open sets. **Definition.** Let \( X \) be a topological space. Subsets \( A, B \) in \( X \) are **separated** if and only if \( \overline{A} \cap B = A \cap \overline{B} = \emptyset \). Thus \( B \) does not contain any limit points of \( A \), and \( A \) does not contain any limit points of \( B \). The notation \( X = A \mid B \) means \( X = A \cup B \) and \( A \) and \( B \) are separated sets. **Theorem 8.1.** The following are equivalent: 1. \( X \) is connected. 2. There is no continuous function \( f : X \rightarrow \mathbb{R}_{\text{std}} \) such that \( f(X) = \{0, 1\} \). 3. \( X \) is not the union of two disjoint non-empty separated sets. 4. \( X \) is not the union of two disjoint non-empty closed sets. 5. The only subsets of \( X \) that are both closed and open in \( X \) are the empty set and \( X \) itself. 6. For every pair of points \( p \) and \( q \) and every open cover \(\{U_{\alpha}\}_{\alpha \in \lambda}\) of \( X \) there exist a finite number of the \( U_{\alpha} \)'s, \(\{U_{\alpha_1}, U_{\alpha_2}, U_{\alpha_3}, \ldots, U_{\alpha_n}\}\) such that \( p \in U_{\alpha_1} \), \( q \in U_{\alpha_n} \), and for each \( i < n \), \( U_{\alpha_i} \cap U_{\alpha_{i+1}} \neq \emptyset \).
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