Obtainable Sizes of Topologies on Finite Sets

Details Obtainable Sizes of Topologies on Finite Sets Obtainable Sizes of Topologies on Finite Sets

2008-02-18

arxiv.org/abs/0802.2550v4

Mathematics

Combinatorics

Final version, to appear in Journal of Combinatorial Theory, Series A

Scientific paper

We study the smallest possible number of points in a topological space having k open sets. Equivalently, this is the smallest possible number of elements in a poset having k order ideals. Using efficient algorithms for constructing a topology with a prescribed size, we show that this number has a logarithmic upper bound. We deduce that there exists a topology on n points having k open sets, for all k in an interval which is exponentially large in n. The construction algorithms can be modified to produce topologies where the smallest neighborhood of each point has a minimal size, and we give a range of obtainable sizes for such topologies.

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