Conditioning random points by the number of vertices of their convex hull: the bi-pointed case
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| Natura: | Preprint |
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2025
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| _version_ | 1866909878228877312 |
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| author | Marckert, Jean-François Morin, Ludovic |
| author_facet | Marckert, Jean-François Morin, Ludovic |
| contents | Pick $N$ random points $U_1,\cdots,U_{N}$ independently and uniformly in a triangle ABC with area 1, and take the convex hull of the set $\{A,B,U_1,\cdots,U_{N}\}$. The boundary of this convex hull is a convex chain $V_0=B,V_1,\cdots,$ $V_{\mathbf{n}(N)}$, $V_{\mathbf{n}(N)+1}=A$ with random size $\mathbf{n}(N)$. The first aim of this paper is to study the asymptotic behavior of this chain, conditional on $\mathbf{n}(N)=n$, when both $n$ and $m=N-n$ go to $+\infty$. We prove a phase transition: if $m=\lfloor nλ\rfloor$ where $λ>0$, this chain converges in probability for the Hausdorff topology to an (explicit) hyperbola ${\cal H}_λ$ as $n\to+\infty$, while, if $m=o(n)$, the limit shape is a parabola. We prove that this hyperbola is solution to an optimization problem: among all concave curves ${\cal C}$ in $ABC$ (incident with $A$ and $B$), ${\cal H}_λ$ is the unique curve maximizing the functional ${\cal C}\mapsto {\sf Area}({\cal C})^λ {\sf L}({\cal C})^3$ where ${\sf L}({\cal C})$ is the affine perimeter of ${\cal C}$. We also give the logarithm expansion of the probability ${\bf Q}^{\triangle \bullet\bullet}_{n,\lfloor nλ\rfloor}$, that $\mathbf{n}(N)=n$ when $N=n+\lfloor nλ\rfloor$.
Take a compact convex set $\mathbf{K}$ with area 1 in the plane, and denote by ${\bf Q}^{\mathbf{K}}_{n,m}$ the probability of the event that the convex hull of $n+m$ iid uniform points in $\mathbf{K}$ is a polygon with $n$ vertices.
We provide some results and conjectures regarding the asymptotic logarithm expansion of ${\bf Q}^{\mathbf{K}}_{n,m}$, as well as results and conjectures concerning limit shape theorems, conditional on this event.
These results and conjectures generalize Bárány's results, who treated the case $λ=0$. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2510_26330 |
| institution | arXiv |
| publishDate | 2025 |
| record_format | arxiv |
| spellingShingle | Conditioning random points by the number of vertices of their convex hull: the bi-pointed case Marckert, Jean-François Morin, Ludovic Probability Combinatorics Differential Geometry 60D05, 52A22 Pick $N$ random points $U_1,\cdots,U_{N}$ independently and uniformly in a triangle ABC with area 1, and take the convex hull of the set $\{A,B,U_1,\cdots,U_{N}\}$. The boundary of this convex hull is a convex chain $V_0=B,V_1,\cdots,$ $V_{\mathbf{n}(N)}$, $V_{\mathbf{n}(N)+1}=A$ with random size $\mathbf{n}(N)$. The first aim of this paper is to study the asymptotic behavior of this chain, conditional on $\mathbf{n}(N)=n$, when both $n$ and $m=N-n$ go to $+\infty$. We prove a phase transition: if $m=\lfloor nλ\rfloor$ where $λ>0$, this chain converges in probability for the Hausdorff topology to an (explicit) hyperbola ${\cal H}_λ$ as $n\to+\infty$, while, if $m=o(n)$, the limit shape is a parabola. We prove that this hyperbola is solution to an optimization problem: among all concave curves ${\cal C}$ in $ABC$ (incident with $A$ and $B$), ${\cal H}_λ$ is the unique curve maximizing the functional ${\cal C}\mapsto {\sf Area}({\cal C})^λ {\sf L}({\cal C})^3$ where ${\sf L}({\cal C})$ is the affine perimeter of ${\cal C}$. We also give the logarithm expansion of the probability ${\bf Q}^{\triangle \bullet\bullet}_{n,\lfloor nλ\rfloor}$, that $\mathbf{n}(N)=n$ when $N=n+\lfloor nλ\rfloor$. Take a compact convex set $\mathbf{K}$ with area 1 in the plane, and denote by ${\bf Q}^{\mathbf{K}}_{n,m}$ the probability of the event that the convex hull of $n+m$ iid uniform points in $\mathbf{K}$ is a polygon with $n$ vertices. We provide some results and conjectures regarding the asymptotic logarithm expansion of ${\bf Q}^{\mathbf{K}}_{n,m}$, as well as results and conjectures concerning limit shape theorems, conditional on this event. These results and conjectures generalize Bárány's results, who treated the case $λ=0$. |
| title | Conditioning random points by the number of vertices of their convex hull: the bi-pointed case |
| topic | Probability Combinatorics Differential Geometry 60D05, 52A22 |
| url | https://arxiv.org/abs/2510.26330 |