Disjoint union topology

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Grade A until it is more presentable.

Definition

Suppose $\big((X_\alpha,\mathcal{J}_\alpha)\big)_{\alpha\in I}$ be an indexed family of topological spaces that are non-empty, the disjoint union topology is a topological space:

• with underlying set $\coprod_{\alpha\in I}X_\alpha$, this is the disjoint union of sets, recall $(x,\beta)\in\coprod_{\alpha\in I}X_\alpha\iff \beta\in I\wedge x\in X_\beta$ and
• The topology where $U\in\mathcal{P}(\coprod_{\alpha\in I}X_\alpha)$ is considered open if and only if $\forall \alpha\in I[X_\alpha\cap U\in\mathcal{J}_\alpha]$ - be sure to notice the abuse of notation going on here.

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TODO: Flesh out notes, mention subspace $X_\alpha\times\{\alpha\}$ and such

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Claim 1: this is indeed a topology

Proof of claims

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Actually surprisingly easy to prove, done on paper. page 1, 7/8/2016, Intro to top manifolds notes. Filed

References

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TODO: Investigate the need to be non-empty, I suspect it's because the union "collapses" in this case, and the space wouldn't be a part of union

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