18 research outputs found
Simplices rarely contain their circumcenter in high dimensions
summary:Acute triangles are defined by having all angles less than , and are characterized as the triangles containing their circumcenter in the interior. For simplices of dimension , acuteness is defined by demanding that all dihedral angles between -dimensional faces are smaller than . However, there are, in a practical sense, too few acute simplices in general. This is unfortunate, since the acuteness property provides good qualitative features for finite element methods. The property of acuteness is logically independent of the property of containing the circumcenter when the dimension is greater than two. In this article, we show that the latter property is also quite rare in higher dimensions. In a natural probability measure on the set of -dimensional simplices, we show that the probability that a uniformly random -simplex contains its circumcenter is
(Bi-)Cohen-Macaulay simplicial complexes and their associated coherent sheaves
Via the BGG correspondence a simplicial complex Delta on [n] is transformed
into a complex of coherent sheaves on P^n-1. We show that this complex reduces
to a coherent sheaf F exactly when the Alexander dual Delta^* is
Cohen-Macaulay. We then determine when both Delta and Delta^* are
Cohen-Macaulay. This corresponds to F being a locally Cohen-Macaulay sheaf.
Lastly we conjecture for which range of invariants of such Delta it must be a
cone.Comment: 16 pages, some minor change