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| Format: | Preprint |
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2022
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| Online Access: | https://arxiv.org/abs/2212.14265 |
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| _version_ | 1866913828922458112 |
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| author | Ruiz, Luis Crespo Santos, Francisco |
| author_facet | Ruiz, Luis Crespo Santos, Francisco |
| contents | Let $Δ_k(n)$ denote the simplicial complex of $(k+1)$-crossing-free subsets of edges in $\binom{[n]}{2}$. Here $k,n\in \mathbb N$ and $n\ge 2k+1$.
Jonsson (2003) proved that (neglecting the short edges that cannot be part of any $(k+1)$-crossing), $Δ_k(n)$ is a shellable sphere of dimension $k(n-2k-1)-1$, and conjectured it to be polytopal. The same result and question arose in the work of Knutson and Miller (2004) on subword complexes.
Despite considerable effort, the only values of $(k,n)$ for which the conjecture is known to hold are $n\le 2k+3$ (Pilaud and Santos, 2012) and $(2,8)$ (Bokowski and Pilaud, 2009).
Using ideas from rigidity theory and choosing points along the moment curve we realize $Δ_k(n)$ as a polytope for $(k,n)\in \{(2,9), (2,10) , (3,10)\}$. We also realize it as a simplicial fan for all $n\le 13$ and arbitrary $k$, except the pairs $(3,12)$ and $(3,13)$.
Finally, we also show that for $k\ge 3$ and $n\ge 2k+6$ no choice of points can realize $Δ_k(n)$ via bar-and-joint rigidity with points along the moment curve or, more generally, via cofactor rigidity with arbitrary points in convex position. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2212_14265 |
| institution | arXiv |
| publishDate | 2022 |
| record_format | arxiv |
| spellingShingle | Realizations of multiassociahedra via rigidity Ruiz, Luis Crespo Santos, Francisco Combinatorics 52B11, 52C25, 52C40 Let $Δ_k(n)$ denote the simplicial complex of $(k+1)$-crossing-free subsets of edges in $\binom{[n]}{2}$. Here $k,n\in \mathbb N$ and $n\ge 2k+1$. Jonsson (2003) proved that (neglecting the short edges that cannot be part of any $(k+1)$-crossing), $Δ_k(n)$ is a shellable sphere of dimension $k(n-2k-1)-1$, and conjectured it to be polytopal. The same result and question arose in the work of Knutson and Miller (2004) on subword complexes. Despite considerable effort, the only values of $(k,n)$ for which the conjecture is known to hold are $n\le 2k+3$ (Pilaud and Santos, 2012) and $(2,8)$ (Bokowski and Pilaud, 2009). Using ideas from rigidity theory and choosing points along the moment curve we realize $Δ_k(n)$ as a polytope for $(k,n)\in \{(2,9), (2,10) , (3,10)\}$. We also realize it as a simplicial fan for all $n\le 13$ and arbitrary $k$, except the pairs $(3,12)$ and $(3,13)$. Finally, we also show that for $k\ge 3$ and $n\ge 2k+6$ no choice of points can realize $Δ_k(n)$ via bar-and-joint rigidity with points along the moment curve or, more generally, via cofactor rigidity with arbitrary points in convex position. |
| title | Realizations of multiassociahedra via rigidity |
| topic | Combinatorics 52B11, 52C25, 52C40 |
| url | https://arxiv.org/abs/2212.14265 |