Dilworth's Theorem - Extension To Infinite Partially Ordered Sets

Extension To Infinite Partially Ordered Sets

Dilworth's theorem for infinite partially ordered sets states that a partially ordered set has finite width w if and only if may be partitioned into w chains. For, suppose that an infinite partial order P has width w, meaning that there are at most a finite number w of elements in any antichain. For any subset S of P, a decomposition into w chains (if it exists) may be described as a coloring of the incomparability graph of S (a graph that has the elements of S as vertices, with an edge between every two incomparable elements) using w colors; every color class in a proper coloring of the incomparability graph must be a chain. By the assumption that P has width w, and by the finite version of Dilworth's theorem, every finite subset S of P has a w-colorable incomparability graph. Therefore, by the De Bruijn–Erdős theorem, P itself also has a w-colorable incomparability graph, and thus has the desired partition into chains (Harzheim 2005).

However, the theorem does not extend so simply to partially ordered sets in which the width, and not just the cardinality of the set, is infinite. In this case the size of the largest antichain and the minimum number of chains needed to cover the partial order may be very different from each other. In particular, for every infinite cardinal number κ there is an infinite partially ordered set of width ℵ₀ whose partition into the fewest chains has κ chains (Harzheim 2005).

Perles (1963) discusses analogs of Dilworth's theorem in the infinite setting.