Proof That a Discrete Space is Separable if and Only if it is Countable


A discrete spaceis separable if and only if it is countable.


Letbe a space with the discrete topology. Every subset ofis both open and closed so the only dense subset ofisitself. Hencecontains a countable dense subset if and only ifis countable. This means that any discrete spaceis separable if and only if is countable.

Alsowith the cofinte topology is separable. Supposeis countable thenis a countable dense subset ofSuppose then thatis not countable, thencontains a non - finite countable subsetIn the cofinite topology the only closed sets are the finite sets and Hence the closure ofis the entire spaceso

Sinceis countableis

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