English
A set s is dense in X if every point of X lies in the closure of s.
Русский
Множество s плотно в X если каждый точка пространства X принадлежит closure(s).
LaTeX
$$$\text{Dense}(s) := \forall x\in X, x\in \operatorname{closure}(s)$$$
Lean4
/-- A set is dense in a topological space if every point belongs to its closure. -/
def Dense (s : Set X) : Prop :=
∀ x, x ∈ closure s