(definition)
We say that is an open cover of , if and if is open .
Example
The set of open balls of any radius around all points in is an open cover.
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(definition)
We say that ∪i∈λ Ai is an open cover of C, if C⊆∪i∈λ Ai and if Ai is open ∀i∈λ.
Example
The set of open balls of any radius around all points in C is an open cover.