 ## Information on Ideals of Partially Ordered Sets

A partially ordered set P = (<,S) is a reflexive, transitive relation on a set S. Another name for partially ordered set is poset. In the Object Server we assume that the set S consists of [n] = {1,2,...,n}, the first n integers. An ideal, I, of a poset P is a subset of the elements of P that satisfies the following property: If y in I and x < y, then x in I. For the poset whose Hasse diagram is shown above the ideals are {}, {1}, {3}, {1,2}, {1,3}, {3,4}, {1,2,3}, {1,3,4}, {1,2,3,4}.

If Gray code output is requested, then each ideal differs from its predecessor by one or two elements.

It is a #P-complete problem to count the number of ideals of a poset. See ???.

For "lex" order, the algorithm used is from the paper G. Steiner, An Algorithm to Generate the Ideals of a Partial Order, Operations Research Letters, 5 (1986) 317-320. If Gray code output is requested, then the algorithm used is from the paper: G. Pruesse and F. Ruskey, Gray Codes from Antimatroids, Order, 10 (1993) 239-252.

## Ideals of Special Posets

### Disjoint chains

If the poset consists of k disjoint chains, where the i-th chain contains ni elements, then the number of ideals is (1+n1)(1+n2) ··· (1+nk).

### The fence poset

The fence poset is defined by the relations (i-1,i), (i+1,i) for odd i. The number of ideals of a n element fence poset is given by the n-th Fibonacci number, Fn.

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