### Abstract

We present extremal constructions connected with the property of simplicial collapsibility. (1) For each (Formula presented.), there are collapsible (and shellable) simplicial d-complexes with only one free face. Also, there are non-evasive d-complexes with only two free faces (both results are optimal in all dimensions). (2) Optimal discrete Morse vectors need not be unique. We explicitly construct a contractible, but non-collapsible 3-dimensional simplicial complex with face vector (Formula presented.) that admits two distinct optimal discrete Morse vectors, (1, 1, 1, 0) and (1, 0, 1, 1). Indeed, we show that in every dimension (Formula presented.) there are contractible, non-collapsible simplicial d-complexes that have (Formula presented.) and (Formula presented.) as distinct optimal discrete Morse vectors. (3) We give a first explicit example of a (non-PL) 5-manifold, with face vector (Formula presented.) 495912, 383136, 110880), that is collapsible but not homeomorphic to a ball. Furthermore, we discuss possible improvements and drawbacks of random approaches to collapsibility and discrete Morse theory. We will introduce randomized versions random-lex-first and random-lex-last of the lex-first and lex-last discrete Morse strategies of Benedetti and Lutz (Exp Math 23(1):66–94, 2014), respectively—and we will see that in many instances the random-lex-last strategy works significantly better than Benedetti–Lutz’s (uniform) random strategy. On the theoretical side, we prove that after repeated barycentric subdivisions, the discrete Morse vectors found by randomized algorithms have, on average, an exponential (in the number of barycentric subdivisions) number of critical cells asymptotically almost surely.

Original language | English (US) |
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Pages (from-to) | 1-30 |

Number of pages | 30 |

Journal | Discrete and Computational Geometry |

DOIs | |

State | Accepted/In press - Feb 17 2017 |

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### Keywords

- Collapsibility
- Library of triangulations
- Random discrete Morse theory
- Shellability

### ASJC Scopus subject areas

- Theoretical Computer Science
- Geometry and Topology
- Discrete Mathematics and Combinatorics
- Computational Theory and Mathematics

### Cite this

*Discrete and Computational Geometry*, 1-30. https://doi.org/10.1007/s00454-017-9860-4