Finite Playground

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Dilworth Theorem and equivalence to Konig-Egervary Theorem

Dilworth’s Theorem. If P is a finite poset, then the maximum size of an antichain in P equals the minimum number of chains needed to cover the elements of P.

Proof. Induction on |P|. We separate into two cases.

(a) Some maximum antichain A omits both a maximal element and a minimal element of P. Then neither D[A] or U[A] contains all of P; the maximality of A implies P = D[A] \cup U[A]. Both D[A] and U[A] has width w(P) because they contain A, and they only intersect in A. Apply induction hypothesis on both sets, we obtain k chains covering D[A] and k chains covering U[A]; combining these chains form k chains covering P.

(b) Every maximum antichain of P consists of all maximal elements or all minimal elements. Thus w(P-\{x,y\}) \le k-1 if x is a minimal element and y is a maximal element below x. Apply induction hypothesis on P-\{x,y\} gives us k-1 chains; add chain \{x,y\} to get a covering of P.

Theorem.  Dilworth’s Theorem is equivalent to Konig-Egervary Theorem.

Proof.  Dilworth to Konig-Egervary: View an n-node bipartite graph G as a bipartite poset. The nodes of one part are maximal elements, and nodes of the other part are minimal. Every covering of the poset of size n-k uses k chains of size 2, which is actually a matching. Each antichain of size n-k corresponds to an independent set in G, and the rest of k nodes forms a vertex cover.

Konig-Egervary to Dilworth: Consider an arbitrary poset P of size n. Define a bipartite graph G as follow: For each element x \in P, create two nodes x^- and x^+. The two parts of the graph are \{x^- : x \in P\} and \{x^+ : x \in P\}. The edge set is \{x^- y^+ : x <_P y\}.

Every matching in G yields a chain-covering in P as follow: We start we n chain, each contains a unique element. If x^- y^+ is in the matching then x and y are combined in the same chain. Therefore because each node in G appears in at most on edge of the matching, a matching of k edges forms a covering of n-k chains.

Given a vertex cover R in G of size k, define a corresponding antichain A = \{x \in P : x^-, x^+ \notin R\}; this is indeed an antichain because if there is a relation x <_P y in A then edge x^- y^+ will be presented, and vertex cover R needs to contain at least one of x^-, y^+.

Claim. No minimal vertex cover of G uses both \{x^-,x^+\}, because by transitivity the sets \{z^- : z \in D(x)\} and \{y^+ : y \in U(x)\} induce a complete bipartite subgraph in G. A vertex cover of a complete bipartite graph must use all of one part. Since x^-,x^+ have all the neighbors in these two sets, we can remove one of x^-,x^+ that is not in the right part and decrease the size of the vertex cover.

Thus a minimum vertex cover of size k yields an antichain of size n-k.