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Going down in (semi)lattices of finite Moore families and convex geometries

Bordalo Gabriela, Caspard Nathalie, Monjardet Bernard (2009)

Czechoslovak Mathematical Journal

In this paper we first study what changes occur in the posets of irreducible elements when one goes from an arbitrary Moore family (respectively, a convex geometry) to one of its lower covers in the lattice of all Moore families (respectively, in the semilattice of all convex geometries) defined on a finite set. Then we study the set of all convex geometries which have the same poset of join-irreducible elements. We show that this set—ordered by set inclusion—is a ranked join-semilattice and we...

Gradedness of the set of rook placements in A n - 1

Mikhail V. Ignatev (2021)

Communications in Mathematics

A rook placement is a subset of a root system consisting of positive roots with pairwise non-positive inner products. To each rook placement in a root system one can assign the coadjoint orbit of the Borel subgroup of a reductive algebraic group with this root system. Degenerations of such orbits induce a natural partial order on the set of rook placements. We study combinatorial structure of the set of rook placements in A n - 1 with respect to a slightly different order and prove that this poset is...

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