dimacs reu 2018 project: sphere packings and number theoryreu.dimacs.rutgers.edu › ~zacharys ›...
TRANSCRIPT
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DIMACS REU 2018Project: Sphere Packings and Number Theory
Alisa Cui, Devora Chait, Zachary StierMentor: Prof. Alex Kontorovich
July 13, 2018
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Apollonian Circle Packing
This is an Apollonian circle packing:
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Apollonian Circle Packing
Here’s how we construct it:
I Start with three mutuallytangent circles
I Draw two more circles, eachof which is tangent to theoriginal three
![Page 4: DIMACS REU 2018 Project: Sphere Packings and Number Theoryreu.dimacs.rutgers.edu › ~zacharys › finalpresentation.pdf · Hyperbolic Geometries I These geometries have several models](https://reader033.vdocuments.net/reader033/viewer/2022042322/5f0c06e77e708231d4336413/html5/thumbnails/4.jpg)
Apollonian Circle Packing
Here’s how we construct it:
I Start with three mutuallytangent circles
I Draw two more circles, eachof which is tangent to theoriginal three
![Page 5: DIMACS REU 2018 Project: Sphere Packings and Number Theoryreu.dimacs.rutgers.edu › ~zacharys › finalpresentation.pdf · Hyperbolic Geometries I These geometries have several models](https://reader033.vdocuments.net/reader033/viewer/2022042322/5f0c06e77e708231d4336413/html5/thumbnails/5.jpg)
Apollonian Circle Packing
Here’s how we construct it:
I Start with three mutuallytangent circles
I Draw two more circles, eachof which is tangent to theoriginal three
![Page 6: DIMACS REU 2018 Project: Sphere Packings and Number Theoryreu.dimacs.rutgers.edu › ~zacharys › finalpresentation.pdf · Hyperbolic Geometries I These geometries have several models](https://reader033.vdocuments.net/reader033/viewer/2022042322/5f0c06e77e708231d4336413/html5/thumbnails/6.jpg)
Apollonian Circle Packing
I Start with three mutuallytangent circles
I Draw two more circles, eachof which is tangent to theoriginal three
![Page 7: DIMACS REU 2018 Project: Sphere Packings and Number Theoryreu.dimacs.rutgers.edu › ~zacharys › finalpresentation.pdf · Hyperbolic Geometries I These geometries have several models](https://reader033.vdocuments.net/reader033/viewer/2022042322/5f0c06e77e708231d4336413/html5/thumbnails/7.jpg)
Apollonian Circle Packing
I Start with three mutuallytangent circles
I Draw two more circles, eachof which is tangent to theoriginal three
I Continue drawing tangentcircles, densely filling space
![Page 8: DIMACS REU 2018 Project: Sphere Packings and Number Theoryreu.dimacs.rutgers.edu › ~zacharys › finalpresentation.pdf · Hyperbolic Geometries I These geometries have several models](https://reader033.vdocuments.net/reader033/viewer/2022042322/5f0c06e77e708231d4336413/html5/thumbnails/8.jpg)
Apollonian Circle Packing
These two images actually represent the same circle packing!
We can go from one realization to the other using circleinversions.
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Apollonian Circle Packing
These two images actually represent the same circle packing!
We can go from one realization to the other using circleinversions.
![Page 10: DIMACS REU 2018 Project: Sphere Packings and Number Theoryreu.dimacs.rutgers.edu › ~zacharys › finalpresentation.pdf · Hyperbolic Geometries I These geometries have several models](https://reader033.vdocuments.net/reader033/viewer/2022042322/5f0c06e77e708231d4336413/html5/thumbnails/10.jpg)
Circle InversionsCircle inversion sends points at a distance of rd from the center ofthe mirror circle to a distance of r/d from the center of the mirrorcircle.
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Circle InversionsCircle inversion sends points at a distance of rd from the center ofthe mirror circle to a distance of r/d from the center of the mirrorcircle.
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Circle Inversions
Circle inversion sends points at a distance of rd from the center ofthe mirror circle to a distance of r/d from the center of the mirrorcircle.
I We apply circle inversions toboth spheres and planes,where planes are consideredspheres of infinite radius.
I Circle inversions preservetangencies and angles.
Source: Malin Christersson
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Apollonian Circle Packing
![Page 14: DIMACS REU 2018 Project: Sphere Packings and Number Theoryreu.dimacs.rutgers.edu › ~zacharys › finalpresentation.pdf · Hyperbolic Geometries I These geometries have several models](https://reader033.vdocuments.net/reader033/viewer/2022042322/5f0c06e77e708231d4336413/html5/thumbnails/14.jpg)
Apollonian Circle Packing
To generate a packing, invert the blue line about the reds
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Apollonian Circle Packing
To generate a packing, invert the blue line about the reds
![Page 16: DIMACS REU 2018 Project: Sphere Packings and Number Theoryreu.dimacs.rutgers.edu › ~zacharys › finalpresentation.pdf · Hyperbolic Geometries I These geometries have several models](https://reader033.vdocuments.net/reader033/viewer/2022042322/5f0c06e77e708231d4336413/html5/thumbnails/16.jpg)
Apollonian Circle Packing
To generate a packing, invert the blue line about the reds
![Page 17: DIMACS REU 2018 Project: Sphere Packings and Number Theoryreu.dimacs.rutgers.edu › ~zacharys › finalpresentation.pdf · Hyperbolic Geometries I These geometries have several models](https://reader033.vdocuments.net/reader033/viewer/2022042322/5f0c06e77e708231d4336413/html5/thumbnails/17.jpg)
Apollonian Circle Packing
To generate a packing, invert the blue line about the reds
![Page 18: DIMACS REU 2018 Project: Sphere Packings and Number Theoryreu.dimacs.rutgers.edu › ~zacharys › finalpresentation.pdf · Hyperbolic Geometries I These geometries have several models](https://reader033.vdocuments.net/reader033/viewer/2022042322/5f0c06e77e708231d4336413/html5/thumbnails/18.jpg)
Apollonian Circle Packing
To generate a packing, invert the blue line about the reds
![Page 19: DIMACS REU 2018 Project: Sphere Packings and Number Theoryreu.dimacs.rutgers.edu › ~zacharys › finalpresentation.pdf · Hyperbolic Geometries I These geometries have several models](https://reader033.vdocuments.net/reader033/viewer/2022042322/5f0c06e77e708231d4336413/html5/thumbnails/19.jpg)
Apollonian Circle Packing
To generate a packing, invert the blue line about the reds
![Page 20: DIMACS REU 2018 Project: Sphere Packings and Number Theoryreu.dimacs.rutgers.edu › ~zacharys › finalpresentation.pdf · Hyperbolic Geometries I These geometries have several models](https://reader033.vdocuments.net/reader033/viewer/2022042322/5f0c06e77e708231d4336413/html5/thumbnails/20.jpg)
Apollonian Circle Packing
To generate a packing, invert the blue line about the reds
![Page 21: DIMACS REU 2018 Project: Sphere Packings and Number Theoryreu.dimacs.rutgers.edu › ~zacharys › finalpresentation.pdf · Hyperbolic Geometries I These geometries have several models](https://reader033.vdocuments.net/reader033/viewer/2022042322/5f0c06e77e708231d4336413/html5/thumbnails/21.jpg)
Apollonian Circle Packing
To generate a packing, invert the blue line about the reds
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Sphere Packings: Definition
The sphere packings we’ve examined this summer areconfigurations where the spheres:
I have varying radii
I are oriented to have mutually disjoint interiors
I densely fill up space
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Hyperbolic Geometries
I There is a surprising connection between sphere packings andnon-Euclidean geometries.
I Euclidean geometry is characterized by Euclid’s parallelpostulate, which states that the angles formed by two linesintersecting on one side of a third line sum to be less than πradians.
Source: Wikipedia
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Hyperbolic Geometries
I There is a surprising connection between sphere packings andnon-Euclidean geometries.
I Euclidean geometry is characterized by Euclid’s parallelpostulate, which states that the angles formed by two linesintersecting on one side of a third line sum to be less than πradians.
Source: Wikipedia
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Hyperbolic Geometries
I These geometries have several models which are each used asis necessary.
I For now, we are going to focus on the upper half-spacemodel of Hn+1: consider Rn+1, subject to x0 > 0. This spacehas its own metric, and has as its boundary Rn.
I Because of the different metric, planes in Hn+1 are actuallyhemispheres, with their circumferences lying in Rn (i.e., thesubset x0 = 0).
I Conveniently, we’ve already been looking at spheres lying inRn! We can “continue our configurations upwards” in what isknown as the Poincare extension.
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Hyperbolic Geometries
I These geometries have several models which are each used asis necessary.
I For now, we are going to focus on the upper half-spacemodel of Hn+1: consider Rn+1, subject to x0 > 0. This spacehas its own metric, and has as its boundary Rn.
I Because of the different metric, planes in Hn+1 are actuallyhemispheres, with their circumferences lying in Rn (i.e., thesubset x0 = 0).
I Conveniently, we’ve already been looking at spheres lying inRn! We can “continue our configurations upwards” in what isknown as the Poincare extension.
![Page 27: DIMACS REU 2018 Project: Sphere Packings and Number Theoryreu.dimacs.rutgers.edu › ~zacharys › finalpresentation.pdf · Hyperbolic Geometries I These geometries have several models](https://reader033.vdocuments.net/reader033/viewer/2022042322/5f0c06e77e708231d4336413/html5/thumbnails/27.jpg)
Poincare Extension
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Hyperbolic Geometries
Another useful model of hyperbolic space is the two-sheetedhyperboloid model.
Resting in R3
Source: supermath.info
I A quadratic form Q is apolynomial where each termis of degree exactly 2. It canbe used to define an innerproduct space.
I We’re working on the topsheet of this 2-sheetedhyperboloid model ofhyperbolic space, where allvectors v satisfy〈v , v〉Q = −1
Where did this quadratic form Q = −1 come from? Circleinversions!
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Hyperbolic Geometries
Another useful model of hyperbolic space is the two-sheetedhyperboloid model.
Resting in R3
Source: supermath.info
I A quadratic form Q is apolynomial where each termis of degree exactly 2. It canbe used to define an innerproduct space.
I We’re working on the topsheet of this 2-sheetedhyperboloid model ofhyperbolic space, where allvectors v satisfy〈v , v〉Q = −1
Where did this quadratic form Q = −1 come from? Circleinversions!
![Page 30: DIMACS REU 2018 Project: Sphere Packings and Number Theoryreu.dimacs.rutgers.edu › ~zacharys › finalpresentation.pdf · Hyperbolic Geometries I These geometries have several models](https://reader033.vdocuments.net/reader033/viewer/2022042322/5f0c06e77e708231d4336413/html5/thumbnails/30.jpg)
Hyperbolic Geometries
Another useful model of hyperbolic space is the two-sheetedhyperboloid model.
Resting in R3
Source: supermath.info
I A quadratic form Q is apolynomial where each termis of degree exactly 2. It canbe used to define an innerproduct space.
I We’re working on the topsheet of this 2-sheetedhyperboloid model ofhyperbolic space, where allvectors v satisfy〈v , v〉Q = −1
Where did this quadratic form Q = −1 come from?
Circleinversions!
![Page 31: DIMACS REU 2018 Project: Sphere Packings and Number Theoryreu.dimacs.rutgers.edu › ~zacharys › finalpresentation.pdf · Hyperbolic Geometries I These geometries have several models](https://reader033.vdocuments.net/reader033/viewer/2022042322/5f0c06e77e708231d4336413/html5/thumbnails/31.jpg)
Hyperbolic Geometries
Another useful model of hyperbolic space is the two-sheetedhyperboloid model.
Resting in R3
Source: supermath.info
I A quadratic form Q is apolynomial where each termis of degree exactly 2. It canbe used to define an innerproduct space.
I We’re working on the topsheet of this 2-sheetedhyperboloid model ofhyperbolic space, where allvectors v satisfy〈v , v〉Q = −1
Where did this quadratic form Q = −1 come from? Circleinversions!
![Page 32: DIMACS REU 2018 Project: Sphere Packings and Number Theoryreu.dimacs.rutgers.edu › ~zacharys › finalpresentation.pdf · Hyperbolic Geometries I These geometries have several models](https://reader033.vdocuments.net/reader033/viewer/2022042322/5f0c06e77e708231d4336413/html5/thumbnails/32.jpg)
From Circle Inversions to Quadratic Forms
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From Circle Inversions to Quadratic Forms
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From Circle Inversions to Quadratic Forms
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From Circle Inversions to Quadratic Forms
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From Circle Inversions to Quadratic Forms
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From Circle Inversions to Quadratic Forms
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From Circle Inversions to Quadratic Forms
d =1
|z | − r− 1
|z |+ r
d =2r
|z |2 − r2
r =r
|z |2 − r2
|z |2 − r2 =r
r
|z |2
r2− 1 =
1
r r= bb
bb− |bz|2 = −1
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Crystallographic Sphere Packings
I First introduced by Kontorovich & Nakamura in 2017
I A crystallographic sphere packing is generated by theaction of a geometrically finite reflection group
I Geometrically finite: generated by a finite number offundamental reflections
I Groups that are geometrically finite have a finite fundamentalpolytope, or the region bounded by the planes associated withtheir fundamental reflections
I The fundamental polytope encodes the same information as aCoxeter diagram
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Crystallographic Sphere Packings
I First introduced by Kontorovich & Nakamura in 2017I A crystallographic sphere packing is generated by the
action of a geometrically finite reflection group
I Geometrically finite: generated by a finite number offundamental reflections
I Groups that are geometrically finite have a finite fundamentalpolytope, or the region bounded by the planes associated withtheir fundamental reflections
I The fundamental polytope encodes the same information as aCoxeter diagram
![Page 41: DIMACS REU 2018 Project: Sphere Packings and Number Theoryreu.dimacs.rutgers.edu › ~zacharys › finalpresentation.pdf · Hyperbolic Geometries I These geometries have several models](https://reader033.vdocuments.net/reader033/viewer/2022042322/5f0c06e77e708231d4336413/html5/thumbnails/41.jpg)
Crystallographic Sphere Packings
I First introduced by Kontorovich & Nakamura in 2017I A crystallographic sphere packing is generated by the
action of a geometrically finite reflection groupI Geometrically finite: generated by a finite number of
fundamental reflections
I Groups that are geometrically finite have a finite fundamentalpolytope, or the region bounded by the planes associated withtheir fundamental reflections
I The fundamental polytope encodes the same information as aCoxeter diagram
![Page 42: DIMACS REU 2018 Project: Sphere Packings and Number Theoryreu.dimacs.rutgers.edu › ~zacharys › finalpresentation.pdf · Hyperbolic Geometries I These geometries have several models](https://reader033.vdocuments.net/reader033/viewer/2022042322/5f0c06e77e708231d4336413/html5/thumbnails/42.jpg)
Crystallographic Sphere Packings
I First introduced by Kontorovich & Nakamura in 2017I A crystallographic sphere packing is generated by the
action of a geometrically finite reflection groupI Geometrically finite: generated by a finite number of
fundamental reflectionsI Groups that are geometrically finite have a finite fundamental
polytope, or the region bounded by the planes associated withtheir fundamental reflections
I The fundamental polytope encodes the same information as aCoxeter diagram
![Page 43: DIMACS REU 2018 Project: Sphere Packings and Number Theoryreu.dimacs.rutgers.edu › ~zacharys › finalpresentation.pdf · Hyperbolic Geometries I These geometries have several models](https://reader033.vdocuments.net/reader033/viewer/2022042322/5f0c06e77e708231d4336413/html5/thumbnails/43.jpg)
Crystallographic Sphere Packings
I First introduced by Kontorovich & Nakamura in 2017I A crystallographic sphere packing is generated by the
action of a geometrically finite reflection groupI Geometrically finite: generated by a finite number of
fundamental reflectionsI Groups that are geometrically finite have a finite fundamental
polytope, or the region bounded by the planes associated withtheir fundamental reflections
I The fundamental polytope encodes the same information as aCoxeter diagram
![Page 44: DIMACS REU 2018 Project: Sphere Packings and Number Theoryreu.dimacs.rutgers.edu › ~zacharys › finalpresentation.pdf · Hyperbolic Geometries I These geometries have several models](https://reader033.vdocuments.net/reader033/viewer/2022042322/5f0c06e77e708231d4336413/html5/thumbnails/44.jpg)
Coxeter Diagram
A Coxeter diagram is a collection of nodes and edges thatrepresents a geometric relationship between n-dimensional spheresand hyperplanes. For two nodes i , j , the edge ei ,j is defined by thefollowing:
ei ,j =
a dotted line, if i and j are disjoint
a thick line, if i and j are tangent
m − 2 thin lines, if the angle between i and j is π/m
no line, if i ⊥ j
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Computation of the Coxeter Diagram
1
2
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Computation of the Coxeter Diagram
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3 4
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Computation of the Coxeter Diagram
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3 4
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Computation of the Coxeter Diagram
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2
3 4
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Computation of the Coxeter Diagram
1
2
3 4
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Computation of the Coxeter Diagram
1
2
3 4
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Computation of the Coxeter Diagram
1
2
3 4
5
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Computation of the Coxeter Diagram
1
2
3 4
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Computation of the Coxeter Diagram
1
2
3 4
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Computation of the Coxeter Diagram
1
2
3 4
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Computation of the Coxeter Diagram
1
2
3 4
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Computation of the Coxeter Diagram
1
2
3 4
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Cluster and CoclusterIn a Coxeter diagram, we select nodes that are connected to eachother only by thick or dashed lines, and to the rest by thick ordashed lines, or no lines at all. For instance:
1 23 4
5
6
789
1 23 4
5
6
789
1 23 4
5
6
789
1 23 4
5
6
789
1 23 4
5
6
789
In each case, the selected nodes form the isolated cluster, and theremainder is the cocluster.
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Cluster and Cocluster
The cocluster acts on the cluster through sphere inversions.
Eerilyenough, we get packings!
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Cluster and Cocluster
The cocluster acts on the cluster through sphere inversions. Eerilyenough, we get packings!
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Cluster and Cocluster
The cocluster acts on the cluster through sphere inversions. Eerilyenough, we get packings!
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Structure Theorem
This is no coincidence.
In 2017, Kontorovich and Nakamuraproved the Structure Theorem for crystallographic packings: aCoxeter diagram’s isolated cluster generates a crystallographicpacking in this manner, and all crystallographic packings arise asthe orbit of an isolated cluster.
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Structure Theorem
This is no coincidence. In 2017, Kontorovich and Nakamuraproved the Structure Theorem for crystallographic packings: aCoxeter diagram’s isolated cluster generates a crystallographicpacking in this manner, and all crystallographic packings arise asthe orbit of an isolated cluster.
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Finiteness Theorem
Why are crystallographic sphere packings a pressing topic?Recently, Kontorovich and Nakamura proved that there existfinitely many crystallographic packings. In fact, no such packingsexist in higher than 21 dimensions.
This means that crystallographic packings can be systematicallyexplored and classified – which was a large part of our research thissummer.
There are 3 sources that can be used to generate crystallographicpackings, and each of us focused on one source:
I Alisa – Polyhedra
I Devora – Bianchi groups
I Zack – Higher dimensional quadratic forms
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Finiteness Theorem
Why are crystallographic sphere packings a pressing topic?Recently, Kontorovich and Nakamura proved that there existfinitely many crystallographic packings. In fact, no such packingsexist in higher than 21 dimensions.
This means that crystallographic packings can be systematicallyexplored and classified – which was a large part of our research thissummer.
There are 3 sources that can be used to generate crystallographicpackings, and each of us focused on one source:
I Alisa – Polyhedra
I Devora – Bianchi groups
I Zack – Higher dimensional quadratic forms
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Finiteness Theorem
Why are crystallographic sphere packings a pressing topic?Recently, Kontorovich and Nakamura proved that there existfinitely many crystallographic packings. In fact, no such packingsexist in higher than 21 dimensions.
This means that crystallographic packings can be systematicallyexplored and classified – which was a large part of our research thissummer.
There are 3 sources that can be used to generate crystallographicpackings, and each of us focused on one source:
I Alisa – Polyhedra
I Devora – Bianchi groups
I Zack – Higher dimensional quadratic forms
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Sources of Circle Packings
Polyhedra Dimension n > 3 Bianchi groups
Apply K-A-T Theorem Select quadratic form
Apply Vinberg’s algorithm
Obtain fundamental polyhedron
Describe with Coxeter diagram
Apply Structure Theorem
Generate circle packing
Which are integral?
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Polyhedra
I How can circle packings arise from polyhedra?
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Polyhedra: Koebe-Andreev-Thurston Theorem
I Theorem: Every polyhedron (up to combinatorialequivalence) has a midsphere.
I Combinatorial equivalence: containing the same informationabout faces, edges, and vertices, regardless of physicalrepresentation
I Midsphere: a sphere tangent to every edge in a polyhedron
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Polyhedra: Koebe-Andreev-Thurston Theorem
I Theorem: Every polyhedron (up to combinatorialequivalence) has a midsphere.
I Combinatorial equivalence: containing the same informationabout faces, edges, and vertices, regardless of physicalrepresentation
I Midsphere: a sphere tangent to every edge in a polyhedron
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Polyhedra: Koebe-Andreev-Thurston Theorem
I Theorem: Every polyhedron (up to combinatorialequivalence) has a midsphere.
I Combinatorial equivalence: containing the same informationabout faces, edges, and vertices, regardless of physicalrepresentation
I Midsphere: a sphere tangent to every edge in a polyhedron
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Polyhedra: Koebe-Andreev-Thurston Theorem
I The midsphere gives rise to two sets of circles: facet circles(purple) and vertex horizon circles (pink)
Planar representation of a polyhedron (left), its vertex horizon circles(center), and its realization with midsphere, vertex horizon circles, andfacet circles (right).
Source: David Eppstein 2004
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Polyhedra: Koebe-Andreev-Thurston Theorem
I Stereographically projecting the facet and vertex horizoncircles onto R2 yields a collection of circles in the plane.
I Stereographic projections map a sphere onto the plane,preserving tangencies and angles
Source: Strebe
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Polyhedra: Koebe-Andreev-Thurston Theorem
I Stereographically projecting the facet and vertex horizoncircles onto R2 yields a collection of circles in the plane.
I Stereographic projections map a sphere onto the plane,preserving tangencies and angles
Source: David E. Joyce 2002
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Polyhedra: Koebe-Andreev-Thurston Theorem
I By K-A-T, this collection of circles is unique up to circleinversions
I These circles actually generate a packing: let pink → cluster,purple → cocluster
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Polyhedra: Koebe-Andreev-Thurston Theorem
I By K-A-T, this collection of circles is unique up to circleinversions
I These circles actually generate a packing: let pink → cluster,purple → cocluster
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Polyhedra: Koebe-Andreev-Thurston Theorem
I By K-A-T, this collection of circles is unique up to circleinversions
I These circles actually generate a packing: let pink → cluster,purple → cocluster
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Polyhedra: Generating a Packing
By constructing the Coxeter diagram of this cluster/coclustergroup, we can see that the Structure Theorem applies
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Polyhedra: Generating a Packing
By constructing the Coxeter diagram of this cluster/coclustergroup, we can see that the Structure Theorem applies
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Polyhedra: Generating a Packing
By constructing the Coxeter diagram of this cluster/coclustergroup, we can see that the Structure Theorem applies
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Polyhedra: Generating a Packing
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Polyhedra: Methods
I Polyhedron data was generated with plantri, a programcreated by Brinkmann and McKay
I We wrote code in Mathematica using some techniques fromZiegler 2004
I Data is being collected and presented on our website
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Polyhedra: Website
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Polyhedra: Findings
Interested in which polyhedra give rise to integral packings
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Polyhedra: Findings
I Previously known integral polyhedra: tetrahedron, squarepyramid, hexagonal pyramid, and gluings thereof
I Define a gluing operation to be a joining along vertices orfaces
I Since the tetrahedron, square pyramid, and hexagonalpyramid cannot be decomposed (unglued) into smallerintegral polyhedra, they can be considered seed polyhedra
I We found a new integral seed polyhedron!
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Polyhedra: Findings
I Previously known integral polyhedra: tetrahedron, squarepyramid, hexagonal pyramid, and gluings thereof
I Define a gluing operation to be a joining along vertices orfaces
I Since the tetrahedron, square pyramid, and hexagonalpyramid cannot be decomposed (unglued) into smallerintegral polyhedra, they can be considered seed polyhedra
I We found a new integral seed polyhedron!
![Page 86: DIMACS REU 2018 Project: Sphere Packings and Number Theoryreu.dimacs.rutgers.edu › ~zacharys › finalpresentation.pdf · Hyperbolic Geometries I These geometries have several models](https://reader033.vdocuments.net/reader033/viewer/2022042322/5f0c06e77e708231d4336413/html5/thumbnails/86.jpg)
Polyhedra: Findings
I Previously known integral polyhedra: tetrahedron, squarepyramid, hexagonal pyramid, and gluings thereof
I Define a gluing operation to be a joining along vertices orfaces
I Since the tetrahedron, square pyramid, and hexagonalpyramid cannot be decomposed (unglued) into smallerintegral polyhedra, they can be considered seed polyhedra
I We found a new integral seed polyhedron!
![Page 87: DIMACS REU 2018 Project: Sphere Packings and Number Theoryreu.dimacs.rutgers.edu › ~zacharys › finalpresentation.pdf · Hyperbolic Geometries I These geometries have several models](https://reader033.vdocuments.net/reader033/viewer/2022042322/5f0c06e77e708231d4336413/html5/thumbnails/87.jpg)
Polyhedra: Findings
I Previously known integral polyhedra: tetrahedron, squarepyramid, hexagonal pyramid, and gluings thereof
I Define a gluing operation to be a joining along vertices orfaces
I Since the tetrahedron, square pyramid, and hexagonalpyramid cannot be decomposed (unglued) into smallerintegral polyhedra, they can be considered seed polyhedra
I We found a new integral seed polyhedron!
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Polyhedra: Findings - 6v7f 2
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Polyhedra: Findings - 6v7f 2
This is the packing of a hexagonal pyramid; it is in fact the samepacking as 6v7f 2.
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Polyhedra: Findings - 6v7f 2
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Bianchi Groups
Bianchi groups, Bi(m), are the set of 2x2 matrices whose entriesare of the complex form a+b
√−m, and which have determinant 1.
Luigi Bianchi began studying these groups over 100 years ago, in1892... Before they were ever connected to circle packings!
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Bianchi Groups
Bianchi groups, Bi(m), are the set of 2x2 matrices whose entriesare of the complex form a+b
√−m, and which have determinant 1.
Luigi Bianchi began studying these groups over 100 years ago, in1892...
Before they were ever connected to circle packings!
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Bianchi Groups
Bianchi groups, Bi(m), are the set of 2x2 matrices whose entriesare of the complex form a+b
√−m, and which have determinant 1.
Luigi Bianchi began studying these groups over 100 years ago, in1892... Before they were ever connected to circle packings!
![Page 94: DIMACS REU 2018 Project: Sphere Packings and Number Theoryreu.dimacs.rutgers.edu › ~zacharys › finalpresentation.pdf · Hyperbolic Geometries I These geometries have several models](https://reader033.vdocuments.net/reader033/viewer/2022042322/5f0c06e77e708231d4336413/html5/thumbnails/94.jpg)
Bianchi Groups
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Bianchi Groups
Figure: Bi(2): From 1892 to 2018
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Bianchi Groups
Bianchi was interested in exploring which Bianchi groups arereflective, meaning finitely generated. 120 years later, Belolipetskyand McLeod conclusively showed that there are a finite number ofthese reflective Bianchi groups, and enumerated them.
The reflective Bianchi groups can be used to generate circlepackings. But how do we go from matrices to circles?
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Bianchi Groups
Bianchi was interested in exploring which Bianchi groups arereflective, meaning finitely generated. 120 years later, Belolipetskyand McLeod conclusively showed that there are a finite number ofthese reflective Bianchi groups, and enumerated them.
The reflective Bianchi groups can be used to generate circlepackings. But how do we go from matrices to circles?
![Page 98: DIMACS REU 2018 Project: Sphere Packings and Number Theoryreu.dimacs.rutgers.edu › ~zacharys › finalpresentation.pdf · Hyperbolic Geometries I These geometries have several models](https://reader033.vdocuments.net/reader033/viewer/2022042322/5f0c06e77e708231d4336413/html5/thumbnails/98.jpg)
Bianchi Groups
Polyhedra Dimension n > 3 Bianchi groups
Apply K-A-T Theorem Select quadratic form
Apply Vinberg’s algorithm
Obtain fundamental polyhedron
Describe with Coxeter diagram
Apply Structure Theorem
Generate circle packing
Which are integral?
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Bianchi Groups
Polyhedra Dimension n > 3 Bianchi groups
Apply K-A-T Theorem Select quadratic form
Apply Vinberg’s algorithm
Obtain fundamental polyhedron
Describe with Coxeter diagram
Apply Structure Theorem
Generate circle packing
Which are integral?
McLeod, Vinberg
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Bianchi Groups
This summer, using McLeod’s application of Vinberg’s algorithm,we catalogued all known circle packings that arise from Bianchigroups.
![Page 101: DIMACS REU 2018 Project: Sphere Packings and Number Theoryreu.dimacs.rutgers.edu › ~zacharys › finalpresentation.pdf · Hyperbolic Geometries I These geometries have several models](https://reader033.vdocuments.net/reader033/viewer/2022042322/5f0c06e77e708231d4336413/html5/thumbnails/101.jpg)
Bianchi Groups
This summer, using McLeod’s application of Vinberg’s algorithm,we catalogued all known circle packings that arise from Bianchigroups.
![Page 102: DIMACS REU 2018 Project: Sphere Packings and Number Theoryreu.dimacs.rutgers.edu › ~zacharys › finalpresentation.pdf · Hyperbolic Geometries I These geometries have several models](https://reader033.vdocuments.net/reader033/viewer/2022042322/5f0c06e77e708231d4336413/html5/thumbnails/102.jpg)
Bianchi Groups
This summer, using McLeod’s application of Vinberg’s algorithm,we catalogued all known circle packings that arise from Bianchigroups.
![Page 103: DIMACS REU 2018 Project: Sphere Packings and Number Theoryreu.dimacs.rutgers.edu › ~zacharys › finalpresentation.pdf · Hyperbolic Geometries I These geometries have several models](https://reader033.vdocuments.net/reader033/viewer/2022042322/5f0c06e77e708231d4336413/html5/thumbnails/103.jpg)
Bianchi Groups and Integrality
An interesting property of Bianchi group circle packings is thatmost of them are integral.
It’s very easy to see that a packing is integral empirically just bycomputing the bends of the packing, but there’s actually a way toprove integrality more rigorously.
Likewise, there’s a way to prove nonintegrality of a packing.
An exciting part of our work this summer is proving integrality andnonintegrality for all known Bianchi packings.
![Page 104: DIMACS REU 2018 Project: Sphere Packings and Number Theoryreu.dimacs.rutgers.edu › ~zacharys › finalpresentation.pdf · Hyperbolic Geometries I These geometries have several models](https://reader033.vdocuments.net/reader033/viewer/2022042322/5f0c06e77e708231d4336413/html5/thumbnails/104.jpg)
Bianchi Groups and Integrality
An interesting property of Bianchi group circle packings is thatmost of them are integral.
It’s very easy to see that a packing is integral empirically just bycomputing the bends of the packing, but there’s actually a way toprove integrality more rigorously.
Likewise, there’s a way to prove nonintegrality of a packing.
An exciting part of our work this summer is proving integrality andnonintegrality for all known Bianchi packings.
![Page 105: DIMACS REU 2018 Project: Sphere Packings and Number Theoryreu.dimacs.rutgers.edu › ~zacharys › finalpresentation.pdf · Hyperbolic Geometries I These geometries have several models](https://reader033.vdocuments.net/reader033/viewer/2022042322/5f0c06e77e708231d4336413/html5/thumbnails/105.jpg)
Bianchi Groups and Integrality
An interesting property of Bianchi group circle packings is thatmost of them are integral.
It’s very easy to see that a packing is integral empirically just bycomputing the bends of the packing, but there’s actually a way toprove integrality more rigorously.
Likewise, there’s a way to prove nonintegrality of a packing.
An exciting part of our work this summer is proving integrality andnonintegrality for all known Bianchi packings.
![Page 106: DIMACS REU 2018 Project: Sphere Packings and Number Theoryreu.dimacs.rutgers.edu › ~zacharys › finalpresentation.pdf · Hyperbolic Geometries I These geometries have several models](https://reader033.vdocuments.net/reader033/viewer/2022042322/5f0c06e77e708231d4336413/html5/thumbnails/106.jpg)
Bianchi Groups and Integrality
An interesting property of Bianchi group circle packings is thatmost of them are integral.
It’s very easy to see that a packing is integral empirically just bycomputing the bends of the packing, but there’s actually a way toprove integrality more rigorously.
Likewise, there’s a way to prove nonintegrality of a packing.
An exciting part of our work this summer is proving integrality andnonintegrality for all known Bianchi packings.
![Page 107: DIMACS REU 2018 Project: Sphere Packings and Number Theoryreu.dimacs.rutgers.edu › ~zacharys › finalpresentation.pdf · Hyperbolic Geometries I These geometries have several models](https://reader033.vdocuments.net/reader033/viewer/2022042322/5f0c06e77e708231d4336413/html5/thumbnails/107.jpg)
Doubling in Bi(3)
One note, which will also be relevant shortly, is that an insight inKontorovich & Nakamura’s 2017 paper was the observation thatwhat was thought to be the Bi(3) Coxeter diagram did notactually represent the full group of mirrors:
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Doubling in Bi(3)
1
2
3 4
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Doubling in Bi(3)
1 2 3 4
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Doubling in Bi(3)
Through a further series of operations, we can transform the
diagram1 2 3 4
into the diagram1 2 3 4 5
.
However, this was done lesssystematically; it primarily derived from looking at the orbit of theoriginal generators acting on themselves until a valid configurationwas found that has an isolated cluster.
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Doubling in Bi(3)
Through a further series of operations, we can transform the
diagram1 2 3 4
into the diagram1 2 3 4 5
. However, this was done lesssystematically; it primarily derived from looking at the orbit of theoriginal generators acting on themselves until a valid configurationwas found that has an isolated cluster.
![Page 112: DIMACS REU 2018 Project: Sphere Packings and Number Theoryreu.dimacs.rutgers.edu › ~zacharys › finalpresentation.pdf · Hyperbolic Geometries I These geometries have several models](https://reader033.vdocuments.net/reader033/viewer/2022042322/5f0c06e77e708231d4336413/html5/thumbnails/112.jpg)
Questions About Existence of Packings
Now that we’ve covered polyhedra and Bianchi groups, which giveall the crystallographic packings in two dimensions, the naturalquestion is: Do higher dimensional packings exist? What can wesay about them?
We can answer this by looking at Coxeterdiagrams of higher-dimensional configurations, and applying theStructure Theorem, which still holds.
![Page 113: DIMACS REU 2018 Project: Sphere Packings and Number Theoryreu.dimacs.rutgers.edu › ~zacharys › finalpresentation.pdf · Hyperbolic Geometries I These geometries have several models](https://reader033.vdocuments.net/reader033/viewer/2022042322/5f0c06e77e708231d4336413/html5/thumbnails/113.jpg)
Questions About Existence of Packings
Now that we’ve covered polyhedra and Bianchi groups, which giveall the crystallographic packings in two dimensions, the naturalquestion is: Do higher dimensional packings exist? What can wesay about them? We can answer this by looking at Coxeterdiagrams of higher-dimensional configurations, and applying theStructure Theorem, which still holds.
![Page 114: DIMACS REU 2018 Project: Sphere Packings and Number Theoryreu.dimacs.rutgers.edu › ~zacharys › finalpresentation.pdf · Hyperbolic Geometries I These geometries have several models](https://reader033.vdocuments.net/reader033/viewer/2022042322/5f0c06e77e708231d4336413/html5/thumbnails/114.jpg)
High-Dimensional Packings
This summer, I worked on putting together the known candidatesfor these packings, namely 41 potential packings for quadratic
forms −dx20 +n∑
i=1x2i for d = 1, 2, 3.
Here is a snapshot of how
some appear on our website:
![Page 115: DIMACS REU 2018 Project: Sphere Packings and Number Theoryreu.dimacs.rutgers.edu › ~zacharys › finalpresentation.pdf · Hyperbolic Geometries I These geometries have several models](https://reader033.vdocuments.net/reader033/viewer/2022042322/5f0c06e77e708231d4336413/html5/thumbnails/115.jpg)
High-Dimensional Packings
This summer, I worked on putting together the known candidatesfor these packings, namely 41 potential packings for quadratic
forms −dx20 +n∑
i=1x2i for d = 1, 2, 3. Here is a snapshot of how
some appear on our website:
![Page 116: DIMACS REU 2018 Project: Sphere Packings and Number Theoryreu.dimacs.rutgers.edu › ~zacharys › finalpresentation.pdf · Hyperbolic Geometries I These geometries have several models](https://reader033.vdocuments.net/reader033/viewer/2022042322/5f0c06e77e708231d4336413/html5/thumbnails/116.jpg)
High-Dimensional Packings
The techniques discussed here have also allowed us to attack thefollowing diagrams that lack isolated clusters:
Source: John McLeod
What’s something that all of these have in common? They all
feature1 2 3 4
as a subdiagram!
![Page 117: DIMACS REU 2018 Project: Sphere Packings and Number Theoryreu.dimacs.rutgers.edu › ~zacharys › finalpresentation.pdf · Hyperbolic Geometries I These geometries have several models](https://reader033.vdocuments.net/reader033/viewer/2022042322/5f0c06e77e708231d4336413/html5/thumbnails/117.jpg)
High-Dimensional Packings
The techniques discussed here have also allowed us to attack thefollowing diagrams that lack isolated clusters:
Source: John McLeod
What’s something that all of these have in common?
They all
feature1 2 3 4
as a subdiagram!
![Page 118: DIMACS REU 2018 Project: Sphere Packings and Number Theoryreu.dimacs.rutgers.edu › ~zacharys › finalpresentation.pdf · Hyperbolic Geometries I These geometries have several models](https://reader033.vdocuments.net/reader033/viewer/2022042322/5f0c06e77e708231d4336413/html5/thumbnails/118.jpg)
High-Dimensional Packings
The techniques discussed here have also allowed us to attack thefollowing diagrams that lack isolated clusters:
Source: John McLeod
What’s something that all of these have in common? They all
feature1 2 3 4
as a subdiagram!
![Page 119: DIMACS REU 2018 Project: Sphere Packings and Number Theoryreu.dimacs.rutgers.edu › ~zacharys › finalpresentation.pdf · Hyperbolic Geometries I These geometries have several models](https://reader033.vdocuments.net/reader033/viewer/2022042322/5f0c06e77e708231d4336413/html5/thumbnails/119.jpg)
High-Dimensional Packings
So, if we apply the known transformation for1 2 3 4
into1 2 3 4 5
followed by a suitable action onthe remainder of the nodes in the Coxeter diagram, then hopefullywe will obtain a valid diagram representing one such desiredsubgroup of mirrors.
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Results
The following Coxeter diagrams were obtained for the n = 6, 7, 8
cases of the quadratic form −3x20 +n∑
i=1x2i :
1 2
3
4
5
6 7
8
9
1
2 3
4
5
67 8
910
1234
5
6
7 89
10 11
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Results
The following is believed to work for n = 10, and works for n = 11:
12
3
4
5 6 7 8
9
10
11
12 13
1415
1
23
45
6
7
89
10
11
12
13
14
15
16
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Results
Lastly, this behemoth is a diagram for n = 13:
1
23
4 5 6
78
9
10
11
12 13 14
15
16
17
18
1920
21
22
23
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References
We are much indebted to the following papers:
I M. Belolipetsky, “Arithmetic Hyperblic Reflection Groups,”2016.
I M. Belolipetsky & J. McLeod, “Reflective andQuasi-Reflective Bianchi Groups,” 2013.
I L. Bianchi, “Sui gruppi di sostituzioni lineari con coefficientiappartenenti a corpi quadratici immaginari,” 1892.
I A. Kontorovich & K. Nakamura, “Geometry and Arithmetic ofCrystallographic Sphere Packings,” 2017.
I J. McLeod, “Arithmetic Hyperbolic Reflection Groups,” 2013.
I E. Vinberg, “On Groups of Unit Elements of CertainQuadratic Forms,” 1972.
I G. Ziegler, “Convex Polytopes: Extremal Constructions andf -Vector Shapes,” 2004.
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Acknowledgements
We would like to acknowledge support and funding from: NSFDMS-1802119; DIMACS; and the Rutgers MathematicsDepartment. We would also like to thank Kei Nakamura and AliceMark for taking the time to discuss this material with us; and wewould most of all like to thank Professor Kontorovich for hisguidance and mentorship on these projects.