English
On a completely metrizable space, there exists a complete metric compatible with its topology.
Русский
На полностью метризуемом пространстве существует полная метрика, совместимая с его топологией.
LaTeX
$$$\\exists d\\text{ on } X:\\text{MetricSpace }(X,d)\\text{ with topology } \\mathcal T_X\\text{ and } \\text{CompleteSpace }(X,d)$$$
Lean4
/-- Construct on a completely metrizable space a metric (compatible with the topology)
which is complete. -/
noncomputable def completelyMetrizableMetric (X : Type*) [TopologicalSpace X] [h : IsCompletelyMetrizableSpace X] :
MetricSpace X :=
h.complete.choose.replaceTopology h.complete.choose_spec.1.symm