TopologicalSpace.IsTopologicalBasis.open_iff_eq_sUnion

∀ {α : Type u} [t : TopologicalSpace α] {B : Set (Set α)},
  TopologicalSpace.IsTopologicalBasis B → ∀ {u : Set α}, IsOpen u ↔ ∃ S ⊆ B, u = ⋃₀ S

Defined in: Mathlib.Topology.Bases
Cited by: 1 results in Mathlib
Foundations: Depth 68 from the axioms · uses propext, Classical.choice, Quot.sound
Assumes: TopologicalSpace

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