Topological space/Definition

Definition

A topological space is a set

$$X$$ coupled with a "topology", $\mathcal{J}$ on $$X$$ . We denote this by the ordered pair $(X,\mathcal{J})$.

• A topology, $\mathcal{J}$ is a collection of subsets of $X$, $$\mathcal{J}\subseteq\mathcal{P}(X)$$ with the following properties:

1. Both $$\emptyset,X\in\mathcal{J}$$
2. For the collection $$\{U_\alpha\}_{\alpha\in I}\subseteq\mathcal{J}$$ where $$I$$ is any indexing set, $$\cup_{\alpha\in I}U_\alpha\in\mathcal{J}$$ - that is it is closed under union (infinite, finite, whatever - "closed under arbitrary union")
3. For the collection $$\{U_i\}^n_{i=1}\subseteq\mathcal{J}$$ (any finite collection of members of the topology) that $$\cap^n_{i=1}U_i\in\mathcal{J}$$

• We call the elements of $\mathcal{J}$ "open sets", that is $\forall S\in\mathcal{J}[S\text{ is an open set}]$, each $S$ is exactly what we call an 'open set'

As mentioned above we write the topological space as

$$(X,\mathcal{J})$$ ; or just $$X$$ if the topology on $$X$$ is obvious from the context.