Proof of Berge’s strong path partition conjecture for k=2k=2

Berge’s strong path partition conjecture from 1982 generalizes and extends Dilworth’s theorem and the Greene–Kleitman theorem which are well known for partially ordered sets. The conjecture is known to be true for all digraphs only for k=1k=1 (by the Gallai–Milgram theorem) and for k≥λk≥λ (where λλ is the cardinality of the longest path in the graph). The attempts made, so far, to prove the conjecture for other values of kk have yielded proofs for acyclic digraphs, but not for general digraphs. In this paper, we prove the conjecture for k=2k=2 for all digraphs. The proof is constructive and it extends the proof for k=1k=1.