Abstract
Given a class P of finite partially ordered sets (posets), then two counting problems are automatically raised. How many posets in P are there on n elements (enumeration of labelled posets) and what is the number of their isomorphism classes (enumeration of unlabelled posets)?
However, not all classes of posets are easy to enumerate. So, answers to these problems have different forms. If we have a sufficiently strong structure theorem, then we can manipulate the calculations in the algebra of formal power series. The required numbers are then obtained recursively from the resulting generating functions. Examples for this exact counting are labelled and unlabelled series-parallel posets [St], labelled and unlabelled interval orders [Ha], labelled and unlabelled graded posets [Kℓ1], [Kℓ2] and labelled tiered posets [Kr]. If we cannot carry out the exact counting then we content ourselves with an asymptotic estimate as in the case of unlabelled 2-dimensional posets [ES]. For other classes we have much cruder estimates. We have an asymptotic estimate for the logarithm of the number of labelled n-element posets [KR2]. For the logarithm of the number of n-element lattices we have a lower bound [KL] and an upper bound [KW] which differ by a multiplicative factor.
The purpose of this paper is to survey these results together with the techniques used in this area.
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© 1989 Kluwer Academic Publishers
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El-Zahar, M.H. (1989). Enumeration of Ordered Sets. In: Rival, I. (eds) Algorithms and Order. NATO ASI Series, vol 255. Springer, Dordrecht. https://doi.org/10.1007/978-94-009-2639-4_9
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DOI: https://doi.org/10.1007/978-94-009-2639-4_9
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