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.
Read more about this topic: Dilworth's Theorem
Famous quotes containing the words extension, infinite, partially, ordered and/or sets:
“Predatory capitalism created a complex industrial system and an advanced technology; it permitted a considerable extension of democratic practice and fostered certain liberal values, but within limits that are now being pressed and must be overcome. It is not a fit system for the mid- twentieth century.”
—Noam Chomsky (b. 1928)
“No person can be considered as possessing a good education without religion. A good education is that which prepares us for our future sphere of action and makes us contented with that situation in life in which God, in his infinite mercy, has seen fit to place us, to be perfectly resigned to our lot in life, whatever it may be.”
—Ann Plato (1820–?)
“Some men have a necessity to be mean, as if they were exercising a faculty which they had to partially neglect since early childhood.”
—F. Scott Fitzgerald (1896–1940)
“I am aware that I have been on many a man’s premises, and might have been legally ordered off, but I am not aware that I have been in many men’s houses.”
—Henry David Thoreau (1817–1862)
“Wilson adventured for the whole of the human race. Not as a servant, but as a champion. So pure was this motive, so unflecked with anything that his worst enemies could find, except the mildest and most excusable, a personal vanity, practically the minimum to be human, that in a sense his adventure is that of humanity itself. In Wilson, the whole of mankind breaks camp, sets out from home and wrestles with the universe and its gods.”
—William Bolitho (1890–1930)