EPSRC Reference: 
EP/V008250/1 
Title: 
Systoles and eigenvalues in nonpositive curvature 
Principal Investigator: 
Fortier Bourque, Mr M 
Other Investigators: 

Researcher CoInvestigators: 

Project Partners: 

Department: 
School of Mathematics & Statistics 
Organisation: 
University of Glasgow 
Scheme: 
New Investigator Award 
Starts: 
01 June 2021 
Ends: 
31 May 2023 
Value (£): 
249,020

EPSRC Research Topic Classifications: 
Algebra & Geometry 
Logic & Combinatorics 

EPSRC Industrial Sector Classifications: 
No relevance to Underpinning Sectors 


Related Grants: 

Panel History: 

Summary on Grant Application Form 
This project will investigate which shapes optimize certain geometric problems. A classical example is the isoperimetric inequality: among all closed curves of a given length, the circle is the one which encloses the largest area. Another example is the sphere packing problem asking for the most efficient way to pack balls of equal radii in Euclidean space. Back in 1611, Kepler conjectured that the usual pyramidal configuration one sees in grocery stores was optimal, but in took until 1998 for a proof to be found by Thomas Hales. Surprisingly, the optimal configurations of spheres are also known in dimensions 8 and 24, thanks to recent breakthroughs by Maryna Viazovska and her coauthors.
The isosystolic problem, popularized by Marcel Berger in the 1970's, asks which Riemannian metrics of unit volume on a given manifold maximize the systole (defined as the length of the shortest noncontractible closed curve in the manifold). There very few cases in which the answer is known: for the 2dimensional torus, the projective plane, and the Klein bottle. More progress can be made by restricting the classes of metrics under consideration. For example, more optimizers are known among constant curvature metrics on some other surfaces and on higherdimensional tori. Although this is not immediately obvious, there is a close connection between the isosystolic problem for tori and the sphere packing problem. Indeed, any torus gives rise to a periodic packing of Euclidean space with spheres of radius equal to half the systole. The larger the systole is, the denser the associated sphere packing is.
The closely related kissing number problem asks what is the largest possible number of homotopically distinct systoles (shortest closed curves) among a given family of Riemannian metrics. The name comes from the fact that for a flat torus, this number counts how many spheres are tangent to (or kiss) any given sphere in the corresponding sphere packing of Euclidean space.
These two problems have analogues in the world of spectral geometry, which studies the frequencies at which a given planar membrane, or drum, can vibrate. The lowest of these frequencies, or the bass note, plays the role of the systole while its multiplicity, or the number of distinct vibration modes producing that note, plays the role of the kissing number. Mathematically, these frequencies are the eigenvalues of the Laplacian operator on the membrane. In particular, these concepts make sense not only for planar membranes, but for any Riemannian manifold.
Both the isosystolic problem and the kissing number problem, as well as their spectral counterparts, can be formulated for regular graphs. There again, a small collection of optimizers is known. However, the main unresolved questions concern the growth of these invariants in terms of the size of the graphs.
The goal of this project is improve the best known bounds for the isosystolic and kissing number problems and their spectral analogues for hyperbolic surfaces. We also aim to generalize results known for flat tori, hyperbolic manifolds and regular graphs to the setting of nonpositively curved spaces.

Key Findings 
This information can now be found on Gateway to Research (GtR) http://gtr.rcuk.ac.uk

Potential use in nonacademic contexts 
This information can now be found on Gateway to Research (GtR) http://gtr.rcuk.ac.uk

Impacts 
Description 
This information can now be found on Gateway to Research (GtR) http://gtr.rcuk.ac.uk 
Summary 

Date Materialised 


Sectors submitted by the Researcher 
This information can now be found on Gateway to Research (GtR) http://gtr.rcuk.ac.uk

Project URL: 

Further Information: 

Organisation Website: 
http://www.gla.ac.uk 