# Relation between Product and Box Topology

## Product and Box Topology

### Product Topology

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

Let $\XX$ be the cartesian product of $\family {X_i}_{i \mathop \in I}$:

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

For each $i \in I$, let $\pr_i: \XX \to X_i$ denote the $i$th projection on $\XX$:

$\forall \family {x_j}_{j \mathop \in I} \in \XX: \map {\pr_i} {\family {x_j}_{j \mathop \in I} } = x_i$

The product topology on $\XX$ is defined as the initial topology $\tau$ on $\XX$ with respect to $\family {\pr_i}_{i \mathop \in I}$.

That is, $\tau$ is the topology generated by:

$\SS = \set {\pr_i^{-1} \sqbrk U: i \in I, U \in \tau_i}$

where $\pr_i^{-1} \sqbrk U$ denotes the preimage of $U$ under $\pr_i$.

### Box Topology

Let $\family {\struct {X_i, \tau_i}}_{i \mathop \in I}$ be an $I$-indexed family of topological spaces.

Let $X$ be the cartesian product of $\family {X_i}_{i \mathop \in I}$, that is:

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

Define:

$\ds \BB := \set {\prod_{i \mathop \in I} U_i: \forall i \in I: U_i \in \tau_i}$

Then $\BB$ is a synthetic basis on $X$, as shown on Basis for Box Topology.

The box topology on $X$ is defined as the topology $\tau$ generated by the synthetic basis $\BB$.

## Relation between Product and Box Topology

### Product Topology

#### Natural Basis of Product Topology

for all $i \in I : U_i \in \tau_i$
for all but finitely many indices $i : U_i = X_i$

$\BB = \set{\ds\prod_{k \mathop = 1}^n U_k : \forall k : U_k \in \tau_k}$