On the Cryptomorphism between Davis' Subset Lattices, Atomic Lattices, and Closure Systems under T1 Separation Axiom
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In this paper we count set closure systems (also known as Moore families) for the case when all single element sets are closed. In particular, we give the numbers of such strict (empty set included) and non-strict families for the base set of size n=6. We also provide the number of such inequivalent Moore families with respect to all permutations of the base set up to n=6. The search in OEIS and existing literature revealed the coincidence of the found numbers with the entry for D. M. Davis' set union lattice (A235604, up to n=5) and |ℒ_n|, the number of atomic lattices on n atoms, obtained by S. Mapes (up to n=6), respectively. Thus we study all those cases, establish one-to-one correspondences between them via Galois adjunctions and Formal Concept Analysis, and provide the reader with two of our enumerative algorithms as well as with the results of these algorithms used for additional tests. Other results include the largest size of intersection free families for n=6 plus our conjecture for n=7, an upper bound for the number of atomic lattices ℒ_n, and some structural properties of ℒ_n based on the theory of extremal lattices.
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