Theorem 7.33. Let X and Y be topological spaces. The product topology on X × Y is the coarsest topology on X × Y that makes the projection maps tx,Ty on X × Y continuous.

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**Definition**. Suppose \( \mathcal{T} \) and \( \mathcal{T}' \) are two topologies on the same underlying set \( X \). If \( \mathcal{T} \subset \mathcal{T}' \), then we say \( \mathcal{T}' \) is finer than \( \mathcal{T} \). Alternatively, we say \( \mathcal{T} \) is coarser than \( \mathcal{T}' \). We say strictly coarser or strictly finer if additionally \( \mathcal{T} \neq \mathcal{T}' \).

**Definition**. A function \( f : X \to Y \) is an embedding if and only if \( f : X \to f(X) \) is a homeomorphism from \( X \) to \( f(X) \), where \( f(X) \) has the subspace topology from \( Y \).

**Definition**. The **projection maps** \( \pi_X : X \times Y \to X \) and \( \pi_Y : X \times Y \to Y \) are defined by \( \pi_X(x, y) = x \) and \( \pi_Y(x, y) = y \).

**Theorem 7.32**. Let \( X \) and \( Y \) be topological spaces. The projection maps \( \pi_X, \pi_Y \) on \( X \times Y \) are continuous, surjective, and open.

In fact, the topology on the product space can be characterized as the coarsest topology that makes the projection maps continuous.

**Theorem 7.33**. Let \( X \) and \( Y \) be topological spaces. The product topology on \( X \times Y \) is the coarsest topology on \( X \times Y \) that makes the projection maps \( \pi_X, \pi_Y \) on \( X \times Y \) continuous.
Transcribed Image Text:**Definition**. Suppose \( \mathcal{T} \) and \( \mathcal{T}' \) are two topologies on the same underlying set \( X \). If \( \mathcal{T} \subset \mathcal{T}' \), then we say \( \mathcal{T}' \) is finer than \( \mathcal{T} \). Alternatively, we say \( \mathcal{T} \) is coarser than \( \mathcal{T}' \). We say strictly coarser or strictly finer if additionally \( \mathcal{T} \neq \mathcal{T}' \). **Definition**. A function \( f : X \to Y \) is an embedding if and only if \( f : X \to f(X) \) is a homeomorphism from \( X \) to \( f(X) \), where \( f(X) \) has the subspace topology from \( Y \). **Definition**. The **projection maps** \( \pi_X : X \times Y \to X \) and \( \pi_Y : X \times Y \to Y \) are defined by \( \pi_X(x, y) = x \) and \( \pi_Y(x, y) = y \). **Theorem 7.32**. Let \( X \) and \( Y \) be topological spaces. The projection maps \( \pi_X, \pi_Y \) on \( X \times Y \) are continuous, surjective, and open. In fact, the topology on the product space can be characterized as the coarsest topology that makes the projection maps continuous. **Theorem 7.33**. Let \( X \) and \( Y \) be topological spaces. The product topology on \( X \times Y \) is the coarsest topology on \( X \times Y \) that makes the projection maps \( \pi_X, \pi_Y \) on \( X \times Y \) continuous.
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