English
A set S is connected if it is nonempty and preconnected.
Русский
Множество S связано, если оно непусто и предсоединено.
LaTeX
$$$$ \\operatorname{IsConnected}(S) \\;\\Longleftrightarrow\\; S \\neq \\varnothing \\wedge \\operatorname{IsPreconnected}(S). $$$$
Lean4
/-- A connected set is one that is nonempty and where there is no non-trivial open partition. -/
def IsConnected (s : Set α) : Prop :=
s.Nonempty ∧ IsPreconnected s