Product Topology is Coarsest Topology such that Projections are Continuous

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Let $\mathbb X = \family {\struct {X_i, \tau_i} }_{i \mathop \in I}$ be an indexed family of topological spaces where $I$ is an arbitrary index set.

Let $X$ be the cartesian product of $\mathbb X$:

$\ds X := \prod_{i \mathop \in I} X_i$

Let $\tau$ be the product topology on $X$.

For each $i \in I$, let $\pr_i : X \to X_i$ be the corresponding projection which maps each ordered tuple in $X$ to the corresponding element in $X_i$:

$\forall x \in X: \map {\pr_i} x = x_i$

Then $\tau$ is the coarsest topology on $X$ such that all the $\pr_i$ are continuous.


The result follows from the definition of the product topology and Equivalence of Definitions of Initial Topology.