“…Duality Theorem was proved by H.G. Kellerer [47], using a functional analytic method, in a more general setting (the proof by the minmax principle is given in [81], see also [38] for the recent development). When c(x, y) → ∞ (|y − x| → ∞), Selection Lemma which is useful in the stochastic optimal control theory gives a simple proof of the Duality Theorem (see [59]).…”