Subspace of Product Space is Homeomorphic to Factor Space

Theorem
Let $\left \langle {\left({X_i, \vartheta_i}\right)}\right \rangle_{i \in I}$ be a family of topological spaces where $I$ is an arbitrary index set.

Consider $T = \left({X, \tau}\right) = {\displaystyle \prod_{i\in I} \left({X_i, \vartheta_i}\right)}$

Suppose that $X$ is non-empty.

Then for each $i \in I$ there is a subspace $Y_i\subseteq X$ which is homeomorphic to $T$

Specifically, for any $z \in X$, let
 * $Y_i = \left\{{ x \in X: \forall j \in I\setminus\{i\}:x_j = z_j }\right\}$

and let $\upsilon_i$ be the subspace topology of $Y_i$ relative to $\tau$.

Then $\left({Y_i,\upsilon_i}\right)$ is homeomorphic to $\left({X_i,\vartheta_i}\right)$, where the homeomorphism is the restriction of $\operatorname{pr}_i$ to $Y_i$.