Knots in Collapsible and Non-Collapsible Balls

  • Bruno Benedetti
  • Frank H. Lutz
Keywords: knots in triangulations, shellability, local constructibility, non-evasiveness, collapsibility, discrete Morse theory


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.

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