Antichain

Let  be a finite partially ordered set, then an antichain in  is a set of pairwise incomparable elements. Antichains are also called Sperner systems in older literature (Comtet 1974).

For example, consider  to be a family of subsets together with the subset relation (i.e.,  if  is a subset of ). The following table gives the antichains on the set of subsets (i.e., the power set) of the -set  for small .

	antichains
1
2
3

The number of antichains on the -set  for , 1, 2, ..., are 1, 2, 5, 19, 167, ... (OEIS A014466). If the empty set is not considered a valid antichain, then these reduce to 0, 1, 4, 18, 166, ... (OEIS A007153; Comtet 1974, p. 273). The numbers obtained by adding one to OEIS A014466, 2, 3, 6, 20, 168, 7581, 7828354, ... (OEIS A000372), are also frequently encountered (Speciner 1972).

The number of antichains on the -set are equal to the number of monotonic increasing Boolean functions of  variables, and also the number of free distributive lattices with  generators (Comtet 1974, p. 273). Determining these numbers is known as Dedekind's problem, and the numbers in each of these sequences are sometimes called Dedekind numbers.

The partial order width of  is the maximum cardinal number of an antichain in . For a partial order, the size of the longest antichain is called the partial order width . Sperner (1928) proved that the maximum size (and hence the width of the partial order) of an antichain containing  elements is

where  is a binomial coefficient and  is the floor function.

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