Countability Properties Preserved under Projection Mapping

Theorem
Let $\sequence {\struct {S_\alpha, \tau_\alpha} }$ be a sequence of topological spaces.

Let $\ds \struct {S, \tau} = \prod \struct {S_\alpha, \tau_\alpha}$ be the product space of $\sequence {\struct {S_\alpha, \tau_\alpha} }$.

Let $\pr_\alpha: \struct {S, \tau} \to \struct {S_\alpha, \tau_\alpha}$ denote the projection on the $\alpha$ coordinate.

Then $\pr_\alpha$ preserves the following countability properties.

That is, if $\struct {S, \tau}$ has one of the following properties, then each of $\struct {S_\alpha, \tau_\alpha}$ has the same property.


 * Separability


 * First-Countability


 * Second-Countability

Proof
First note that Projection from Product Topology is Continuous.

It follows from Continuous Image of Separable Space is Separable that separability is preserved under projections.

Next note that Projection from Product Topology is Open.

It follows from Countability Axioms Preserved under Open Continuous Surjection that:
 * First-Countability
 * Second-Countability

are preserved under projections.