Z are continuous, then their composition Theorem 7.9. If f : X → Y and g gof : X → Z is continuous. : Y

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ISBN:9780470458365
Author:Erwin Kreyszig
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Could you explain how to show 7.9 in detail?

**Definition.** Let \( X \) and \( Y \) be topological spaces. A function or map \( f : X \rightarrow Y \) is a *continuous function* or *continuous map* if and only if for every open set \( U \) in \( Y \), \( f^{-1}(U) \) is open in \( X \).

**Definition.** Let \( f : X \rightarrow Y \) be a function between topological spaces \( X \) and \( Y \), and let \( x \in X \). Then \( f \) is *continuous at the point* \( x \) if and only if for every open set \( V \) containing \( f(x) \), there is an open set \( U \) containing \( x \) such that \( f(U) \subseteq V \). Thus a function \( f : X \rightarrow Y \) is continuous if and only if it is continuous at each point.

**Theorem 7.9.** If \( f : X \rightarrow Y \) and \( g : Y \rightarrow Z \) are continuous, then their composition \( g \circ f : X \rightarrow Z \) is continuous.
Transcribed Image Text:**Definition.** Let \( X \) and \( Y \) be topological spaces. A function or map \( f : X \rightarrow Y \) is a *continuous function* or *continuous map* if and only if for every open set \( U \) in \( Y \), \( f^{-1}(U) \) is open in \( X \). **Definition.** Let \( f : X \rightarrow Y \) be a function between topological spaces \( X \) and \( Y \), and let \( x \in X \). Then \( f \) is *continuous at the point* \( x \) if and only if for every open set \( V \) containing \( f(x) \), there is an open set \( U \) containing \( x \) such that \( f(U) \subseteq V \). Thus a function \( f : X \rightarrow Y \) is continuous if and only if it is continuous at each point. **Theorem 7.9.** If \( f : X \rightarrow Y \) and \( g : Y \rightarrow Z \) are continuous, then their composition \( g \circ f : X \rightarrow Z \) is continuous.
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