English
If f and g are monotone maps, then x ↦ f(x) ∪ g(x) is monotone.
Русский
Если f и g монотонны, то x ↦ f(x) ∪ g(x) монотонна.
LaTeX
$$$ \operatorname{Monotone}(f) \land \operatorname{Monotone}(g) \Rightarrow \operatorname{Monotone}(x \mapsto f(x) \cup g(x)) $$$
Lean4
theorem union [Preorder β] {f g : β → Set α} (hf : Monotone f) (hg : Monotone g) : Monotone fun x => f x ∪ g x :=
hf.sup hg