PhD student, Mathematical Institute, University of Oxford

Jethro Warnett

I work on interacting particle systems and on the algorithms built from them, mostly on questions of well-posedness, mean-field limits and rates of convergence.

Supervised by Prof. José A. Carrillo and Dr Jakub Skrzeczkowski. warnett@maths.ox.ac.uk

Jethro Warnett standing on a lakeside path with moored sailing boats and forested mountains behind him.
Lago Maggiore

About

I have been a PhD student in mathematics at the Mathematical Institute, University of Oxford since September 2023, supervised by Prof. José A. Carrillo and Dr Jakub Skrzeczkowski. My doctorate is funded by an EPSRC scholarship and I am a member of the Oxford Centre for Nonlinear PDE.

My work is on interacting particle systems and the partial differential equations that describe them when the number of particles is large. Several methods used in practice for optimisation and sampling, including consensus-based optimisation and Stein variational gradient descent, are systems of this type. They are used more widely than the theory currently covers, and I try to close some of that gap: well-posedness, quantitative mean-field limits, and rates of convergence, using tools from nonlinear PDE, optimal transport and functional inequalities.

Before Oxford I did an MSc in Mathematics at ETH Zürich, graduating with distinction, and a BSc in Mathematics at the University of Bern, summa cum laude. From 2019 to 2021 I was a research assistant in the Computer Graphics Group in Bern, working with Prof. David Bommes on combinatorial optimisation for quad meshing.

I teach across the Oxford undergraduate syllabus as a Stipendiary Lecturer at The Queen's College, having held the same post at St Hilda's College before that, and I tutor and demonstrate for the Mathematical Institute.

Away from the desk I cook, take photographs, ride mountain bikes and hike. A Swiss upbringing is hard to shake.

Research

Many current algorithms for optimisation, sampling and learning are built from a large number of particles that interact through a kernel. They are straightforward to implement and they perform well in practice, but the theory describing their behaviour is often incomplete.

I study these methods as nonlinear, nonlocal PDEs and as gradient flows in Wasserstein space. The questions are whether the dynamics are well posed, how closely the mean-field description matches the finite systems that are actually simulated, and how fast the method converges. The last of these usually comes down to a functional inequality.

Interests

Particle methods for optimisation and sampling

Consensus-based optimisation, Stein variational gradient descent and related schemes, whose behaviour is set by the interaction kernel.

Wasserstein gradient flows

Evolution equations read as steepest descent of an energy over probability measures, and what that structure gives in terms of convergence rates.

Mean-field limits

Quantitative estimates linking an N-particle system to its continuum limit, so that results proved in the limit still say something at finite N.

Nonlinear and nonlocal PDEs

Well-posedness and long-time behaviour of kernel-driven aggregation and diffusion equations, including limits in which the kernel concentrates.

Functional inequalities

Log-Sobolev type inequalities, in particular the Stein-log-Sobolev inequality, which control the rate of decay to equilibrium.

Mathematics of machine learning

Analytical foundations of learning algorithms, including current work on a mathematical perspective on transformers.

Publications

Most recent first. Abstracts are taken from the published version or from arXiv.

Published and accepted

  1. 2026

    Well-posedness and mean-field limit estimate of a consensus-based algorithm for multiplayer games

    Hui Huang, Jethro Warnett

    Communications on Pure and Applied Analysis, vol. 31, pp. 146–165, 2026.

    Abstract of Well-posedness and mean-field limit estimate of a consensus-based algorithm for multiplayer games

    Recently, an earlier paper introduces a derivative-free consensus-based particle method that finds the Nash equilibrium of non-convex multiplayer games, where it proves the global exponential convergence in the sense of mean-field law. This paper aims to address theoretical gaps in that work, specifically by providing a quantitative estimate of the mean-field limit with respect to the number of particles, as well as establishing the well-posedness of both the finite particle model and the corresponding mean-field dynamics.

  2. 2023

    Min-Deviation-Flow in Bi-directed Graphs for T-Mesh Quantization

    Martin Heistermann, Jethro Warnett, David Bommes

    ACM Transactions on Graphics, vol. 42, no. 4, article 70, July 2023. Presented at SIGGRAPH 2023.

    Nominated for Best Paper, SIGGRAPH 2023

    Abstract of Min-Deviation-Flow in Bi-directed Graphs for T-Mesh Quantization

    Subdividing non-conforming T-mesh layouts into conforming quadrangular meshes is a core component of state-of-the-art (re-)meshing methods. Typically, the required constrained assignment of integer lengths to T-Mesh edges is left to generic branch-and-cut solvers, greedy heuristics, or a combination of the two. This either does not scale well with input complexity or delivers suboptimal result quality. We introduce the Minimum-Deviation-Flow Problem in bi-directed networks (Bi-MDF) and demonstrate its use in modeling and efficiently solving a variety of T-Mesh quantization problems. We develop a fast approximate solver as well as an iterative refinement algorithm based on matching in graphs that solves Bi-MDF exactly. Compared to the state-of-the-art QuadWild implementation on the authors' 300 dataset, our exact solver finishes after only 0.49% (total 17.06s) of their runtime (3491s) and achieves 11% lower energy while an approximation is computed after 0.09% (3.19s) of their runtime at the cost of 24% increased energy. A novel half-arc-based T-Mesh quantization formulation extends the feasible solution space to include previously unattainable quad meshes. The Bi-MDF problem is more general than our application in layout quantization, potentially enabling similar speedups for other optimization problems that fit into the scheme, such as quad mesh refinement.

Preprints

  1. 2026

    Stein Variational Gradient Descent dynamics for highly concentrated kernels

    José A. Carrillo, Jakub Skrzeczkowski, Jethro Warnett

    arXiv:2605.03627 [math.AP], submitted 5 May 2026.

    Abstract of Stein Variational Gradient Descent dynamics for highly concentrated kernels

    Stein Variational Gradient Descent (SVGD) is a widely used in practice algorithm for scalable sampling with deterministic particle updates. We study its behavior in the singular limit where the kernel bandwidth tends to zero. In this regime, we show that the nonlocal SVGD dynamics converge to a local evolution equation that can be formally interpreted as a Wasserstein gradient flow with quadratic mobility. We analyze this singular limit in two settings: integrable kernels and weighted kernels. In the weighted case, the proof is supported by recently established Stein-log-Sobolev inequalities, which provide the necessary functional control. Overall, our results clarify how SVGD collapses from a nonlocal interacting particle system to a local gradient-flow dynamics as the kernel concentrates.

  2. 2026

    Well-posedness and mean-field limit estimate of a consensus-based algorithm for min-max problems

    Hui Huang, Jethro Warnett

    arXiv:2602.12886 [math.OC], submitted 13 February 2026.

    Abstract of Well-posedness and mean-field limit estimate of a consensus-based algorithm for min-max problems

    The recent work arXiv:2407.17373 proposes a derivative-free consensus-based particle method that computes global solutions to nonconvex-nonconcave min-max problems and establishes global exponential convergence in the sense of the mean-field law. This paper aims to address the theoretical gaps in arXiv:2407.17373, specifically by providing a quantitative estimate of the mean-field limit with respect to the number of particles, as well as establishing the well-posedness of both the finite particle model and the corresponding mean-field dynamics.

  3. 2024

    The Stein-log-Sobolev inequality and the exponential rate of convergence for the continuous Stein variational gradient descent method

    José A. Carrillo, Jakub Skrzeczkowski, Jethro Warnett

    arXiv:2412.10295 [math.AP], submitted 13 December 2024.

    Abstract of The Stein-log-Sobolev inequality and the exponential rate of convergence for the continuous Stein variational gradient descent method

    The Stein Variational Gradient Descent method is a variational inference method in statistics that has recently received a lot of attention. The method provides a deterministic approximation of the target distribution, by introducing a nonlocal interaction with a kernel. Despite the significant interest, the exponential rate of convergence for the continuous method has remained an open problem, due to the difficulty of establishing the related so-called Stein-log-Sobolev inequality. Here, we prove that the inequality is satisfied for each space dimension and every kernel whose Fourier transform has a quadratic decay at infinity and is locally bounded away from zero and infinity. Moreover, we construct weak solutions to the related PDE satisfying exponential rate of decay towards the equilibrium. The main novelty in our approach is to interpret the Stein-Fisher information, also called the squared Stein discrepancy, as a duality pairing between H−1(ℝd) and H1(ℝd), which allows us to employ the Fourier transform. We also provide several examples of kernels for which the Stein-log-Sobolev inequality fails, partially showing the necessity of our assumptions.

CV

A condensed version. The full CV, with complete teaching and travel records, is in the PDF.

Appointments and awards

  • 2025Stipendiary Lecturer, The Queen's College, Oxford
  • 2023Stipendiary Lecturer, St Hilda's College, Oxford
  • 2023EPSRC Scholarship for doctoral study at Oxford
  • 2023Best paper nomination, SIGGRAPH 2023
  • 2023VMP Best Teaching Assistant Award
  • 2024–25Five competitive travel grants: Granada, Hamburg, Santa Barbara, Bonn, Providence

Selected talks

  • 09/2025Gradient Flows Face-to-Face, Granada
  • 09/2025Conference on Mathematics of Machine Learning, Hamburg
  • 07/2025Junior Researcher Workshop in Optimal Transport and Applications, Santa Barbara
  • 07/2025New Perspectives in Nonlocal and Nonlinear PDEs, Anacapri
  • 05/2025Junior Analysis Seminar (invited), Imperial College London
  • 03/2025AER Young Mathematicians Meeting, Regensburg

Teaching

  • OxfordIntroductory Calculus · Numerical Analysis · Integration · Metric Spaces and Complex Analysis · Functional Analysis I
  • ETH ZürichAnalysis I, II and IV (Fourier theory and Hilbert spaces)
  • BernAnalysis I, II · Algebra

Research visits

  • 10/2025Simon Fraser University, Vancouver, hosted by Prof. Razvan Fetecau, for ongoing work on a mathematical perspective on transformers

Available as a postdoc

I am finishing my PhD at Oxford and looking for a postdoctoral position, ideally somewhere that works on particle systems, gradient flows, or the analysis of algorithms.

I have taught alongside research throughout the PhD and would like to continue. I am also open to problems outside my current area, and to collaborations that need analysis of particle systems or optimal transport.

Mobility
High. Has already changed country twice.
Warranty
All claims come with proofs.