
arXiv: 1303.2070
We construct the first explicit example of a simplicial 3-ball $B_{15,66}$ that is not collapsible. It has only 15 vertices. We exhibit a second 3-ball $B_{12,38}$ with 12 vertices that is collapsible and not shellable, but evasive. Finally, we present the first explicit triangulation of a 3-sphere $S_{18, 125}$ (with only 18 vertices) that is not locally constructible. All these examples are based on knotted subcomplexes with only three edges; the knots are the trefoil, the double trefoil, and the triple trefoil, respectively. The more complicated the knot is, the more distant the triangulation is from being polytopal, collapsible, etc. Further consequences of our work are:(1) Unshellable 3-spheres may have vertex-decomposable barycentric subdivisions. (This shows the strictness of an implication proven by Billera and Provan.)(2) For $d$-balls, vertex-decomposable implies non-evasive implies collapsible, and for $d=3$ all implications are strict. (This answers a question by Barmak.)(3) Locally constructible 3-balls may contain a double trefoil knot as a 3-edge subcomplex. (This improves a result of Benedetti and Ziegler.)(4) Rudin's ball is non-evasive.
collapsibility, Computational Geometry (cs.CG), FOS: Computer and information sciences, Shellability for polytopes and polyhedra, knots in triangulations, local constructibility, Triangulating manifolds, Geometric Topology (math.GT), Mathematics - Geometric Topology, shellability, 57Q15, 52B22, 57M25, FOS: Mathematics, Knots and links in the \(3\)-sphere, Mathematics - Combinatorics, Computer Science - Computational Geometry, Combinatorics (math.CO), non-evasiveness, discrete Morse theory
collapsibility, Computational Geometry (cs.CG), FOS: Computer and information sciences, Shellability for polytopes and polyhedra, knots in triangulations, local constructibility, Triangulating manifolds, Geometric Topology (math.GT), Mathematics - Geometric Topology, shellability, 57Q15, 52B22, 57M25, FOS: Mathematics, Knots and links in the \(3\)-sphere, Mathematics - Combinatorics, Computer Science - Computational Geometry, Combinatorics (math.CO), non-evasiveness, discrete Morse theory
| selected citations These citations are derived from selected sources. This is an alternative to the "Influence" indicator, which also reflects the overall/total impact of an article in the research community at large, based on the underlying citation network (diachronically). | 15 | |
| popularity This indicator reflects the "current" impact/attention (the "hype") of an article in the research community at large, based on the underlying citation network. | Top 10% | |
| influence This indicator reflects the overall/total impact of an article in the research community at large, based on the underlying citation network (diachronically). | Top 10% | |
| impulse This indicator reflects the initial momentum of an article directly after its publication, based on the underlying citation network. | Average |
