Simon Willerton

Listen pal, you can't just waltz in here, use my toaster and spout universal truths without qualification!
Hal Hartley - Surviving Desire

I am a pure mathematician interested in various aspects of geometry, topology, category theory, metric spaces and quantum algebra. I am a Catster and a host of the n-Category Café.

Teaching

MAS336 Differential Georemtry

Some graphical experiments

The Catsters

The Catsters consists of my colleague Eugenia Cheng and me. We have put up instructional YouTube videos on Category Theory.

PhD students

Potential PhD students should look at my PhD student page

My PhD student webpage

Contact

My contact details are on my departmental webpage

My departmental webpage

You know that I write slowly. This is chiefly because I am never satisfied until I have said as much as possible in a few words, and writing briefly takes far more time than writing at length.
Karl Friedrich Gauss

Research

Preprints

On the magnitude of odd balls via potential functions
arxiv:1804.02174
Heuristic and computer calculations for the magnitude of metric spaces
arxiv:0910.5500 [Blogpost]

Research Publications

The magnitude of odd balls via Hankel determinants of reverse Bessel polynomials
Discrete Analysis 2020:5, 42 pp. arxiv:1708.03227
Categorifying the magnitude of a graph (with Richard Hepworth)
Homotopy, Homology and Applications 19 (2017) 31-60. arxiv:1505.04125 [Blogpost]
The Legendre-Fenchel transform from a category theoretic perspective
Theory and Applications of Categories (to appear). arxiv:1501.03791. [Blogposts I and II]
Spread: a measure of the size of metric spaces
International Journal of Computational Geometry and Applications, 25 (2015) p. 207. arxiv:1209.2300 [Blogpost]
Tight spans, Isbell completions and semi-tropical modules
Theory and Applications of Categories 28 (2013) 696-732. arxiv:1302.4370 [Blogpost]
On the magnitude of spheres, surfaces and other homogeneous spaces
Geometriae Dedicata (2013). arxiv:1005.4041
On the asymptotic magnitude of subsets of Euclidean space (with Tom Leinster)
Geometriae Dedicata 164 (2013) 287-310. arxiv:0908.1582 [Blogpost]
The Mukai pairing, I: a categorical approach (with A.Caldararu),
New York Journal of Mathematics 16 (2010) 61-98. arXiv:0707.2052
On the Rozansky-Witten weight systems (with J.Roberts),
Algebraic & Geometric Topology 10 (2010) 1455-1519. math.DG/0602653. See also Justin Roberts'talk.
A diagrammatic approach to Hopf monads
[For less wide typesetting style see arXiv version.]
Arabian Journal of Science and Engineering C - Theme Issue "Interactions of algebraic and coalgebraic structures (theory and applications)" December 2008; Vol. 33, Number 2C math/0807.0658
The twisted Drinfeld double of a finite group via gerbes and finite groupoids
Algebraic & Geometric Topology 8 (2008) 1419-1457 math.QA/0503266
Gerbes and Homotopy Quantum Field Theories (with U.Bunke and P.Turner)
Algebr. Geom. Topol. 4 (2004) 407-437 math/0201116
Homotopy quantum field theories and related ideas (with M.Brightwell and P.Turner)
Int. J. Modern Phys. A 18 Supplement (2003) 115-122.
An almost-integral universal Vassiliev invariant of knots
Algebraic and Geometric Topology 2 (2002) paper no. 29, 649-664 math/0105190
On the first two Vassiliev invariants,
Experimental Mathematics (2002).
Free groups and finite type invariants of pure braids (with J.Mostovoy)
Math. Proc. Camb. Phil. Soc. 132 (2001) 117-130.
The Kontsevich integral and algebraic structures on the space of diagrams,
Knots in Hellas '98, Series on Knots and Everything vol 24, World Scientific, 2000, 530-546.
Vassiliev invariants as polynomials,
Knot Theory, Banach Centre Publications 42 (1998) 457-463.
A combinatorial half-integration from weight system to Vassiliev knot invariant,
J. Knot Theory Ramifications, 7 no. 4 (1998) 519-526.
Vassiliev invariants and the Hopf algebra of chord diagrams,
Math. Proc. Camb. Phil. Soc., 119 (1996) 55-65.

Popularizations

A topological tie-in,
Nature 368 103-104, 10 March 1994.

Unpublished

On the Vassiliev invariants for knots and for pure braids,
(official abstract) Edinburgh University PhD Thesis, July 1997. This contains material from the above papers together with ideas on the relationship between Vassiliev invariants for pure braids and de Rham homotopy theory.
Vassiliev invariants for knots,
Essay for Part III of the Cambridge Mathematical Tripos, May 1993. A survey of Bar-Natan's paper On the Vassiliev knot invariants, which contained some additional work of my own.

Selected Beamer Talks

Dualities, enriched categories and metric spaces, November 2014.
Magnitude and other measures of metric spaces, July 2012.
Magnitude of Metric Spaces II, September 2010.
A prequel to this talk, Magnitude of Metric Spaces I, was given by Tom Leinster.
Traces in Low Dimensional Algebra, November 2009.
Measuring metric spaces: short sightedness and population diversity, SoMaS Colloquium, March 2009.
Two 2-traces August 2008.
There are two videos of this talk on YouTube:
- Oxford August 2008
- Birmingham September 2010

Selected writings at the n-Category Café

Optimal Transport and Enriched Categories III: Duality Within Pricing (Aug 7, 2021): Learn how conjugation between optimal prices comes from an enriched adjunction.
Optimal Transport and Enriched Categories II (Jun 19, 2021): Read about how the dual transport problem arises from general linear programming duality (and how that arises from general Lagrangian duality).
Optimal Transport and Enriched Categories I (Jun 6, 2021): Read about the optimal transport problem and its dual.
Torsion: Graph Magnitude Homology Meets Combinatorial Topology (Apr 11, 2018): Work through an example of torsion in magnitude homology based on the work of Kaneta and Yoshinaga.
On the Magnitude Function of Domains in Euclidean Space, II (Mar 25, 2018): Continue reading about the work of Gimperlein and Goffeng
Magnitude Homology Reading Seminar, I (Mar 19, 2018): See the first in a Sheffield seminar series on the Leinster-Shulman paper on magnitude homology.
SageMath and 3D Models in Webpages (Dec 19, 2017): Emded 3d models of surfaces into your websites using SageMath.
Lattice Paths and Continued Fractions II (Sep 22, 2017): Contemplate further continued fractions.
Lattice Paths and Continued Fractions I (Sep 18, 2017): Feel flabbergasted by Flajolet’s Fundamental Lemma
Schröder Paths and Reverse Bessel Polynomials (Aug 28, 2017): See how the reverse Bessel polynomials (which make an appearance in the magnitude of balls) have a combinatorial interpretation.
Instantaneous Dimension of Finite Metric Spaces via Magnitude and Spread (Aug 5, 2017): Download a talk on instantaneous dimension.
Barceló and Carbery on the Magnitude of Odd Balls (Sep 9, 2016): Read about Barcel'o and Carbery’s calculation of the magnitude of odd dimensional balls, utilizing the potential theory developed by Meckes.
Categorifying the Magnitude of a Graph (May 13, 2015): See how there’s a homology theory for graphs with magnitude as its Euler characteristic.
A Scale-Dependent Notion of Dimension for Metric Spaces (Part 1) (Mar 11, 2015): Try to understand how dimension can depend on scale
Mathematics and Magic: the de Bruijn Card Trick (Jan 5, 2015): Perform a magic trick using the power of maths.
Enrichment and the Legendre-Fenchel Transform II (May 22, 2014): Get the second installment of how Legendre-Fenchel duality is an example of the profunctor nucleus construction.
Enrichment and the Legendre--Fenchel Transform I (Apr 16, 2014): Remind yourself about basics of the Legendre–Fenchel transform.
Fuzzy Logic and Enriching Over the Category [0,1] (Mar 15, 2014): Watch me try to understand fuzzy logic from an enriched category theory perspective
Galois Correspondences and Enriched Adjunctions (Feb 5, 2014): Translate from category theory to order theory
Ends (Jan 5, 2014): End your ignorance of ends!
Classical Dualities and Formal Concept Analysis (Sep 12, 2013): Find out what algebraic varieties, convex sets, linear subspaces, real numbers, logical theories and extension fields have in common with formal concepts.
Formal Concept Analysis (Sep 2, 2013): Have a peek at the notion of formal concept analysis
The Nucleus of a Profunctor: Some Categorified Linear Algebra (Aug 19, 2013): Watch some linear algebra being categorified.
Torsors and enriched categories (Jun 3, 2013): Read about a different take on torsors
Project Scheduling and Copresheaves (Mar 24, 2013): Find out what PERT graphs have to do with enriched categories
Tight spans, Isbell completions and semi-tropical modules (Jan 20, 2013): See how these three things are related.
The Spread of a Metric Space (Sep 5, 2012): Read how this notion of size for metric spaces has some interesting properties.
Integral Transforms and the Pull-Push Perspective, I (Nov 7, 2010): Start to see how enriched profunctors can be viewed as categorifications of integral kernels.
Enriching Over a Category of Subsets (Aug 30, 2010): Discover how enriched category theory leads to the definition of some generalized metrics on the space of continuous functions on the unit interval.
On the Magnitude of Spheres, Surfaces and Other Homogeneous Spaces (Apr 21, 2010): See the details of a new paper on the magnitude of metric spaces.
Modeling Surface Diagrams (Mar 24, 2010): Watch some videos to see how I’m trying to make 3d models of categorical surface diagrams.
Intrinsic Volumes and Weyl's Tube Formula (Mar 12, 2010): Read about how the volume of a tube around a surface in 3-space depends only on intrinsic invariants of the surface.
A Look at the Mathematical Origins of Western Musical Scales (Feb 26, 2010): See how the rational numbers 2 and 3/2 gave birth to the Western musical scale.
More Magnitude of Metric Spaces and Problems with Penguins (Oct 10, 2009): Learn about the tenuous link between emperor penguins and the magnitude of metric spaces.

Visit my co-authors' websites: