no goals
α : Type u
β : Type v
γ : Type ?u.61888
δ : Type ?u.61891
ε : Type ?u.61894
ζ : Type ?u.61897
inst✝⁵ : TopologicalSpace α
inst✝⁴ : TopologicalSpace β
inst✝³ : TopologicalSpace γ
inst✝² : TopologicalSpace δ
inst✝¹ : TopologicalSpace ε
inst✝ : TopologicalSpace ζ
s : Set β
⊢ frontier (univ ×ˢ s) = univ ×ˢ frontier s
simp [frontier_prod_eq]