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The blocker A* of an antichain A in a finite poset P is the set of elements minimal with the property of having with each member of A a common predecessor. The following is done: (1) The posets P for which A** = A for all antichains are characterized. (2) The blocker A* of a symmetric antichain in the partition lattice is characterized. (3) Connections with the question of finding minimal size blocking...
It is shown that the neighborhood complexes of a family of vertex critical subgraphs of Kneser graphsthe stable Kneser graphs introduced by L. Schrijverare spheres up to homotopy. Furthermore, it is shown that the neighborhood complexes of a subclass of the stable Kneser graphs contain the boundaries of associahedra (simplicial complexes encoding triangulations of a polygon) as a strong deformation...
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