A D1106: Topological space $T = (X, \mathcal{T})$ is a Kolmogorov topological space if and only if \begin{equation} \forall \, x, y \in X \left( x \neq y \quad \implies \quad \exists \, U \in \mathcal{T} : (x \in U, y \not\in U) \lor (x \not\in U, y \in U) \right) \end{equation}