84 research outputs found
How many -vertex triangulations does the -sphere have?
It is known that the -sphere has at most
combinatorially distinct triangulations with vertices. Here we construct at
least such triangulations.Comment: 9 pages, 1 figur
Combinatorial 3-manifolds with 10 vertices
We give a complete enumeration of all combinatorial 3-manifolds with 10
vertices: There are precisely 247882 triangulated 3-spheres with 10 vertices as
well as 518 vertex-minimal triangulations of the sphere product
and 615 triangulations of the twisted sphere product S^2_\times_S^1.
All the 3-spheres with up to 10 vertices are shellable, but there are 29
vertex-minimal non-shellable 3-balls with 9 vertices.Comment: 9 pages, minor revisions, to appear in Beitr. Algebra Geo
Small examples of non-constructible simplicial balls and spheres
We construct non-constructible simplicial -spheres with vertices
and non-constructible, non-realizable simplicial -balls with vertices
for .Comment: 9 pages, 3 figure
Almost simplicial polytopes: the lower and upper bound theorems
International audiencethis is an extended abstract of the full version. We study n-vertex d-dimensional polytopes with at most one nonsimplex facet with, say, d + s vertices, called almost simplicial polytopes. We provide tight lower and upper bounds for the face numbers of these polytopes as functions of d, n and s, thus generalizing the classical Lower Bound Theorem by Barnette and Upper Bound Theorem by McMullen, which treat the case s = 0. We characterize the minimizers and provide examples of maximizers, for any d
Collapsibility of CAT(0) spaces
Collapsibility is a combinatorial strengthening of contractibility. We relate
this property to metric geometry by proving the collapsibility of any complex
that is CAT(0) with a metric for which all vertex stars are convex. This
strengthens and generalizes a result by Crowley. Further consequences of our
work are:
(1) All CAT(0) cube complexes are collapsible.
(2) Any triangulated manifold admits a CAT(0) metric if and only if it admits
collapsible triangulations.
(3) All contractible d-manifolds () admit collapsible CAT(0)
triangulations. This discretizes a classical result by Ancel--Guilbault.Comment: 27 pages, 3 figures. The part on collapsibility of convex complexes
has been removed and forms a new paper, called "Barycentric subdivisions of
convexes complex are collapsible" (arXiv:1709.07930). The part on enumeration
of manifolds has also been removed and forms now a third paper, called "A
Cheeger-type exponential bound for the number of triangulated manifolds"
(arXiv:1710.00130
On -stellated and -stacked spheres
We introduce the class of -stellated (combinatorial) spheres
of dimension () and compare and contrast it with the
class () of -stacked homology -spheres.
We have , and for . However, for each there are
-stacked spheres which are not -stellated. The existence of -stellated
spheres which are not -stacked remains an open question.
We also consider the class (and ) of
simplicial complexes all whose vertex-links belong to
(respectively, ). Thus, for , while . Let
denote the class of -dimensional complexes all whose
vertex-links are -stacked balls. We show that for , there is a
natural bijection from onto which is the inverse to the boundary map .Comment: Revised Version. Theorem 2.24 is new. 18 pages. arXiv admin note:
substantial text overlap with arXiv:1102.085
A non-partitionable Cohen-Macaulay simplicial complex
A long-standing conjecture of Stanley states that every Cohen-Macaulay
simplicial complex is partitionable. We disprove the conjecture by constructing
an explicit counterexample. Due to a result of Herzog, Jahan and Yassemi, our
construction also disproves the conjecture that the Stanley depth of a monomial
ideal is always at least its depth.Comment: Final version. 13 pages, 2 figure
- …