Definition Tietze_extension_func : point_set X -> point_set RTop.

Theorem Tietze_extension_theorem: forall (X:TopologicalSpace) (F:Ensemble (point_set X)) (f:point_set (SubspaceTopology F) -> point_set RTop), normal_sep X -> closed F -> continuous f -> exists g:point_set X -> point_set RTop, continuous g /\ (forall x:point_set (SubspaceTopology F), g (subspace_inc F x) = f x).