Discrete Geometry

姜子麟 Zilin Jiang

Arizona State University

April 13, 2021

Extremal properties of simple geometric objects

What's the minimum number of distinct distances between $n$ points on a plane?

Erdős 1946 · Moser 1952 · Chung 1984

Chung, Szemerédi & Trotter 1992 · Székely 1993

Solymosi & Tóth 2001 · Tardos 2003 · Katz & Tardos 2004

Guth & Katz 2015

Geometry of ruled surfaces, polynomial method.

Given $n$ points not all collinear in $\mathbb{R}^2$,

what's the minimum number of ordinary lines ?

Sylvester 1893 · Gallai 1944

Dirac & Motzkin 1951 · Kelly & Moser 1958

Csima & Sawyer 1993 · Green & Tao 2012

Menelaus's theorem, Euler formula

Cayley–Bacharach theorem, sum-product estimate

What's the densest way to arrange

non-overlapping spheres in $\mathbb{R}^n$?

Kepler 1611 · Gauss 1831 · Thue 1890

Fejes Tóth 1940 · Hales 1998

Cohn, Kumar, Miller, Radchenko, Viazovska 2016

Poisson summation formula, modular forms, automated proof checking

How high do $n$ satellites need to fly?

Each satellite sees a zone of the sphere

What's the minimum width of $n$ equal zones

covering the unit sphere?

Is it $\pi / 3$ for $n = 3$?

Distribute $n$ great circles to minimize the greatest distance between a point and the nearest great circle.

What's the minimum width of $n$ equal zones

covering the unit sphere?

Fejes Tóth's zone conjecture

The width of $n$ equal zones covering the

unit sphere is at least $\pi / n$.

Research Problems: Exploring a Planet, 1973

Rosta 1972: 3 equal zones

Linhart 1974: 4 equal zones

Fodor, Vígh and Zarnócz 2016: a lower bound

for example, $\pi/6.83$ for 5 equal zones

Fejes Tóth's zone conjecture 1973 · J.–Polyanskii 2017

The total
width of $n$ equal zones covering

the unit sphere is at least $\pi$ in any dimension; Characterization of equality cases.

A plank in $\mathbb{R}^d$ is

Tarski's plank problem

What's the minimum total width of planks covering a given convex body?

What's the minimum total width of planks covering a unit disk?

Plank of width $w$

Planks cover disk

$\sum w_i \ge 2$

Archimedes: Arch of area $\pi w$

Arches cover hemisphere

$\sum \pi w_i \ge 2\pi$

What's the minimum total width of planks

covering a given convex body $C$?

Tarski's plank conjecture 1932 · Bang 1951

If convex body $C$ is covered by planks, then their total width is at least the width of $C$.

A direction of a plank is

Bang's lemma

Can choose directions such that $v_1 + \dots + v_n$ is not covered

Fejes Tóth's zone conjecture 1973 · J.–Polyanskii 2017

The total width of $n$ zones covering the unit sphere is at least $\pi$.

Each zone is a plank $\cap$
the unit sphere

and it has two directions

Bang's lemma: $v := v_1 + \dots + v_n$ is not covered

If $\lVert v \rVert \le 1$, then $\hat{v}$ is not covered.

Otherwise, $v$ is large in magnitude

We merge some zones

Erdős' circle covering problem · Goodman–Goodman 1945

A non-separable family of balls of radii $r_1, \dots, r_n$ can be

covered by a ball of radius $r_1 + \dots + r_n$

Bang's plank conjecture 1951

Total relative width of planks covering

a convex body is at least $1$.

Bezdek's annulus conjecture 2003

Total width of planks covering an annulus with a small hole at center is at least the diameter.

Ball 1992 using Bang's lemma

A lower bound on sphere packing density.

Equiangular lines in $\mathbb{R}^n$ are lines through origin pairwise separated by the same angle

What's the maximum number of

equiangular lines in $\mathbb{R}^n$?

$n$ | 2 | 3-4 | 5 | 6 | 7-13 | 14 | ... | 23-41 | 42 | 43 |

max | 3 | 6 | 10 | 16 | 28 | 28-29 | ... | 276 | 276-288 | 344 |

What's the maximum number of equiangular lines in $\mathbb{R}^n$?

Gerzon 1973

At most $\frac{1}{2}n(n+1)$

de Caen 2000

At least $cn^2$

Angles $\to$ 90° as $n\to\infty$

What happens if the angles are held fixed?

What's the maximum number $E_\alpha(n)$ of

equiangular lines in $\mathbb{R}^n$ with a fixed angle $\arccos\alpha$?

Lemmens, Seidel 1973 | $E_{1/3}(n) = 2(n-1)$ for $n \ge 15$ |

Neumann 1973 | $E_{\alpha}(n) \le 2n$, unless $1/\alpha$ is odd |

Neumaier 1989 | $E_{1/5}(n) = \lfloor \frac{3}{2}(n-1) \rfloor$ for $n \ge n_0$ |

Bukh 2016 | $E_{\alpha}(n) \le c_\alpha n$ |

Balla, Dräxler, Sudakov, Keevash 2018 | $E_{\alpha}(n) \le 1.93n$ for $n \ge n_0(\alpha)$ if $\alpha \neq 1/3$ |

Bukh's conjecture on equiangular lines with fixed angle

$E_{1/(2k-1)}(n) \approx \frac{kn}{k-1}$. $E_{1/7}(n) \approx \frac{4}{3}n$.

What's the maximum number $E_\alpha(n)$ of

equiangular lines in $\mathbb{R}^n$ with a fixed angle $\arccos\alpha$?

Conjecture J.–Polyanskii

$E_{\alpha}(n) \approx \frac{kn}{k-1}$, where $k = k(\lambda)$, $\lambda = \frac{1-\alpha}{2\alpha}$.

Spectral radius order $k(\lambda) := $ smallest $k$ such that

$\exists$ $k$-vertex graph $G$ whose adjacency matrix has spectral radius $= \lambda$

$\alpha$ | $\lambda$ | $G$ | $k$ | $E_\alpha(n)$ |

$\tfrac{1}{3}$ | $1$ | $2$ | $2n$ | |

$\tfrac{1}{5}$ | $2$ | $3$ | $\tfrac{3n}{2}$ | |

$\frac{1}{7}$ | $3$ | $4$ | $\tfrac{4n}{3}$ |

J.–Polyanskii 2018

True for all $\lambda \le \sqrt{2 + \sqrt{5}}$.

What's the maximum number $E_\alpha(n)$ of

equiangular lines in $\mathbb{R}^n$ with a fixed angle $\arccos\alpha$?

me

Jonathan Tidor

Yufei Zhao

Yuan Yao

Shengtong Zhang

J., Tidor, Yao, Zhang, Zhao 2019

$E_\alpha(n) = \lfloor \frac{k}{k-1}(n-1) \rfloor$ for $n \ge n_0(\alpha)$ if $k(\lambda) < \infty$;

$E_\alpha(n) = n+o(n)$ otherwise.

J., Tidor, Yao, Zhang, Zhao 2019

Multiplicity of second largest eigenvalue of an $n$-vertex connected graph of bounded degree is at most $Cn/\log\log n$.

Cayley graph of $\mathrm{PSL}(2,p)$ has

second eigenvalue multiplicity at least $\sqrt[3]{n}$

What is the maximum second eigenvalue multiplicity of

an $n$-vertex connected graph of bounded degree?

What is the maximum second eigenvalue multiplicity of

an $n$-vertex Cayley graph of bounded degree?

Gromov, Colding–Minicozzi, Kleiner

Lee–Makarychev: Constant for abelian groups

What is the maximum number of unit vectors in $\mathbb{R}^n$ such that pairwise inner products are either $\alpha$ or $\beta$?

Zauner's conjecture 1999

Maximum number of equiangular lines in $\mathbb{C}^n$ is $n^2$.