English
For any family of lower sets f i, the complement distributes over the supremum: (⨆ i, f i).compl = ⨆ i, (f i).compl.
Русский
Для произвольной семейной функции нижних множеств справедливо: (⨆ i, f i).compl = ⨆ i, (f i).compl.
LaTeX
$$$ (\big \vee_i f_i).\mathrm{compl} = \big \vee_i (f_i).\mathrm{compl} $$$
Lean4
protected theorem compl_iSup (f : ι → LowerSet α) : (⨆ i, f i).compl = ⨆ i, (f i).compl :=
UpperSet.ext <| by simp only [coe_compl, coe_iSup, compl_iUnion, UpperSet.coe_iSup]