[0JH]Prerequisites:[2BR]↺↻.\(X\) is disconnected if and only if there exist non-empty sets \(A,B⊆ X\) whose union covers \(X\), but such that \(\overline B {\mathbf{d}}A\) and \( B {\mathbf{d}}\overline A\).
[0JJ]↺↻
