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union-closed sets; Frankl’s conjecture; lattice; semimodularity; planar lattice
A lattice $L$ is said to satisfy (the lattice theoretic version of) Frankl’s conjecture if there is a join-irreducible element $f\in L$ such that at most half of the elements $x$ of $L$ satisfy $f\le x$. Frankl’s conjecture, also called as union-closed sets conjecture, is well-known in combinatorics, and it is equivalent to the statement that every finite lattice satisfies Frankl’s conjecture. Let $m$ denote the number of nonzero join-irreducible elements of $L$. It is well-known that $L$ consists of at most $2^m$ elements. Let us say that $L$ is large if it has more than $5\cdot 2^{m-3}$ elements. It is shown that every large semimodular lattice satisfies Frankl’s conjecture. The second result states that every finite semimodular planar lattice $L$ satisfies Frankl’s conjecture. If, in addition, $L$ has at least four elements and its largest element is join-reducible then there are at least two choices for the above-mentioned $f$.
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