# Poppy-seed bagel theorem

In physics, the poppy-seed bagel theorem concerns interacting particles (e.g., electrons) confined to a bounded surface (or body) $A$ when the particles repel each other pairwise with a magnitude that is proportional to the inverse distance between them raised to some positive power $s$ . In particular, this includes the Coulomb law observed in Electrostatics and Riesz potentials extensively studied in Potential theory. For $N$ such particles, an equilibrium (stable) state, which depends on the parameter $s$ , is attained when the associated energy of the system is minimal (the so-called generalized Thomson problem). For large numbers of points, these equilibrium configurations provide a discretization of $A$ which may or may not be nearly uniform with respect to the surface area (or volume) of $A$ . The Poppy-seed bagel theorem asserts that for a large class of sets $A$ , the uniformity property holds when the parameter $s$ is larger than or equal to the dimension of the set $A$ . For example, when the points ("poppy-seeds") are confined to a torus embedded in 3-dimensions (or "surface of a bagel"), one can create a large number of points that are nearly uniformly spread on the surface by imposing a repulsion proportional to the inverse square distance between the points, or any stronger repulsion ($s\geq 2$ ). From a culinary perspective, to create the nearly perfect poppy-seed bagel where bites of equal size anywhere on the bagel would contain essentially the same number of poppy seeds, impose at least an inverse square distance repelling force on the seeds.

## Formal definitions

For a parameter $s>0$ and an $N$ -point set $\omega _{N}=\{x_{1},\ldots ,x_{N}\}\subset \mathbb {R} ^{p}$ , the $s$ -energy of $\omega _{N}$ is defined as follows:

$E_{s}(\omega _{N}):=\sum _{i=1,\ldots ,N}\sum _{\stackrel {j=1,\ldots ,N}{j\not =i}}{\frac {1}{|x_{i}-x_{j}|^{s}}}$ For a compact set $A$ we define its minimal $N$ -point $s$ -energy as
${\mathcal {E}}_{s}(A,N):=\min E_{s}(\omega _{N}),$ where the minimum is taken over all $N$ -point subsets of $A$ ; i.e., $\omega _{N}\subset A$ . Configurations $\omega _{N}$ that attain this infimum are called $N$ -point $s$ -equilibrium configurations.

## Poppy-seed bagel theorem for bodies

We consider compact sets $A\subset \mathbb {R} ^{p}$ with the Lebesgue measure $\lambda (A)>0$ and $s\geqslant p$ . For every $N\geqslant 2$ fix an $N$ -point $s$ -equilibrium configuration $\omega _{N}^{*}=\{x_{1,N},\ldots ,x_{N,N}\}$ . Set

$\mu _{N}:={\frac {1}{N}}\sum _{i=1,\ldots ,N}\delta _{x_{i,N}},$ where $\delta _{x}$ is a unit point mass at point $x$ . Under these assumptions, in the sense of weak convergence of measures,
$\mu _{N}{\stackrel {*}{\rightarrow }}\mu ,$ where $\mu$ is the Lebesgue measure restricted to $A$ ; i.e., $\mu (B)=\lambda (A\cap B)/\lambda (A)$ . Furthermore, it is true that
$\lim _{N\to \infty }{\frac {{\mathcal {E}}_{s}(A,N)}{N^{1+s/p}}}={\frac {C_{s,p}}{\lambda (A)^{s/p}}},$ where the constant $C_{s,p}$ does not depend on the set $A$ and, therefore,
$C_{s,p}=\lim _{N\to \infty }{\frac {{\mathcal {E}}_{s}([0,1]^{p},N)}{N^{1+s/p}}},$ where $[0,1]^{p}$ is the unit cube in $\mathbb {R} ^{p}$ .

## Poppy-seed bagel theorem for manifolds

Near minimal $s$ -energy 1000-point configurations on a torus ($d=2$ ) $s=0.01  $s=1  $s=2=d$ Consider a smooth $d$ -dimensional manifold $A$ embedded in $\mathbb {R} ^{p}$ and denote its surface measure by $\sigma$ . We assume $\sigma (A)>0$ . Assume $s\geqslant d$ As before, for every $N\geqslant 2$ fix an $N$ -point $s$ -equilibrium configuration $\omega _{N}^{*}=\{x_{1,N},\ldots ,x_{N,N}\}$ and set

$\mu _{N}:={\frac {1}{N}}\sum _{i=1,\ldots ,N}\delta _{x_{i,N}}.$ Then, in the sense of weak convergence of measures,
$\mu _{N}{\stackrel {*}{\rightarrow }}\mu ,$ where $\mu (B)=\sigma (A\cap B)/\sigma (A)$ . If $H^{d}$ is the $d$ -dimensional Hausdorff measure, then
$\lim _{N\to \infty }{\frac {{\mathcal {E}}_{s}(A,N)}{N^{1+s/d}}}=2^{s}\alpha _{d}^{-s/d}\cdot {\frac {C_{s,d}}{(H^{d}(A))^{s/d}}},$ where $\alpha _{d}=\pi ^{d/2}/\Gamma (1+d/2)$ is the volume of a d-ball.

## The constant $C_{s,p}$ For $p=1$ , it is known that $C_{s,1}=2\zeta (s)$ , where $\zeta (s)$ is the Riemann zeta function. The following connection between the constant $C_{s,p}$ and the problem of Sphere packing is known:

$\lim _{s\to \infty }(C_{s,p})^{1/s}={\frac {1}{s}}\left({\frac {\alpha _{p}}{\Delta _{p}}}\right)^{1/p},$ where $\alpha _{p}$ is the volume of a p-ball and
$\Delta _{p}=\sup \rho ({\mathcal {P}}),$ where the supremum is taken over all families ${\mathcal {P}}$ of non-overlapping unit balls such that the limit
$\rho ({\mathcal {P}})=\lim _{r\to \infty }{\frac {\lambda \left([-r,r]^{p}\cap \bigcup _{B\in {\mathcal {P}}}B\right)}{(2r)^{p}}}$ exists.