The past year has been difficult for everyone. While members of the group have been in good health, personal lives have suffered and we have had little time to update this blog with what we’ve been up to. Over the next few weeks we will try to provide updates to cover this past year.

# Author: cardiffquest

# New Marie Skłodowska-Curie Fellowship

We are delighted to announce that our team has been awarded another Marie Skłodowska-Curie Fellowship. Frank Rösler, with the supervision of Jonathan Ben-Artzi, has been successful with his proposal entitled “Computational Complexity in Quantum Mechanics” (COCONUT).

The short description provided in the proposal states: *“The goal of this project is to improve our understanding of how to perform computations in quantum mechanics and classify their complexity. This will be achieved by using modern methods from spectral approximation theory in conjunction with the recently introduced Solvability Complexity Index.”*

The total value of the award is €212,934.

# New paper: Uniform convergence in von Neumann’s ergodic theorem in the absence of a spectral gap

**** Update: this is now published in Ergod. Th. & Dynam. Sys. ****

Jonathan Ben-Artzi and Baptiste Morisse recently submitted a paper entitled *Uniform convergence in von Neumann’s ergodic theorem in absence of a spectral gap*.

Von Neumann’s ergodic theorem states that “time” averages converge to “spatial” avergaes: given a one-parameter family of unitary maps the average converges to the projection of onto the space of functions invariant under as .

Generally there is no rate. However, if the generator of has a spectral gap, the rate is . In the present paper, it is shown that even in the absence of a spectral gap one can obtain a rate, albeit on a subspace of , and with a rate worse than . This is done by obtaining a suitable estimate for the density of the spectrum near zero (low frequencies).

# Small Scales and Homogenisation (SmaSH)

We hosted the workshop *Small Scales and Homogenisation (SmaSH)* between 24-26 June 2019. With participants coming from as far as Russia this was a great success.

# New paper: Weak Poincaré inequalities in the absence of spectral gaps

**** Update: this paper is now published in Ann. Henri Poincaré ****

Jonathan Ben-Artzi recently uploaded a new paper entitled *Weak Poincaré inequalities in absence of spectral gaps,* co-authored with Amit Einav.

For Markov semigroups it is well-known that the following are equivalent:

- The generator has a spectral gap,
- The generator satisfies a Poincaré inequality,
- Solutions decay exponentially

In this paper, they study semigroups which **lack a spectral gap** (such as the heat semigroup in ) and try to see how much of the above theorem remains true. They prove that an *estimate on the density of the spectrum* near 0 leads to a **weak Poincaré inequality**, which in turn leads to an **algebraic decay rate**.

This is applied to the heat semigroup, where the optimal decay rate is recovered. In this case, the weak Poincaré inequality is no more than the **Nash inequality**. This is done for the fractional Laplacian as well, with similar results.

# New team member: Frank Rösler

On 1 October 2018 we were joined by a new team member, Dr Frank Rösler. Frank completed his PhD at Durham University and worked as a Research Assistant in Freiburg (Germany).

He is interested in the spectral theory of non-selfadjoint operators and other operator-theoretic questions in PDE theory. His past projects involved pseudospectra of non-normal Schrödinger Operators and more general resolvent norm estimates of partial differential operators. More recently, he studied problems in Asymptotic Analysis and Homogenisation from an operator-theoretic perspective.

Welcome Frank!

# An Analyst, a Geometer and a Probabilist Walk Into a Bar

Last month (25-29 June 2018) we hosted a successful workshop in Cardiff, entitled *An Analyst, a Geometer and a Probabilist Walk Into a Bar*. There were about 40 participants from Austria, Canada, France, Israel, Italy, Portugal, Spain, Switzerland, the UK and USA. These included many students and early career researchers. Here is the workshop photo:

# Postdoc position advertised

We are in the process of hiring another postdoc to join our team (see current members). We are looking for someone with research interests in analysis, and, more precisely, someone who is interested in how to approximate infinite dimensional objects in finite dimensions. A typical example is approximating operators in Hilbert space. If you think this sounds interesting please apply. Of course, you will also be free to continue your own research.

The advert is here.

This will involve joint work with our colleagues Marco Marletta (Cardiff) and Anders Hansen (Cambridge).

# Marie Skłodowska-Curie Fellowship Success

Jonathan Ben-Artzi and Junyong Zhang have been awarded a Marie Skłodowska-Curie Fellowship which will commence on 1 July 2018 for a period of two years. Their project, entitled “Geometric Analysis of Dilute Plasmas” (GRANDPA), will focus on studying regularity theory and long-time behavior of plasmas governed by the Vlasov-Maxwell system. The abstract reads:

*“The ultimate goal of this Fellowship is to understand the long time behaviour of plasmas governed by the relativistic Vlasov- Maxwell system (RVM). The main difficulty is the hyperbolic nature of Maxwell’s equations (the electromagnetic fields propagate at the speed of light): particles that travel close to the speed of light nearly interact with their own fields. It is not currently known whether particles can be accelerated to such speeds, and, if so, whether this necessarily leads to development of singularities. This is a major open problem.”*

The combined expertise of Jonathan and Junyong in kinetic theory and in dispersive equations played a central role in the success of this application. The total value of the award is €195,455.

# New team member: Junyong Zhang

This month we welcomed Dr Junyong Zhang as a research associate. He joins us from the Beijing Institute of Technology, where he maintains his affiliation. Junyong is interested in harmonic analysis, spectral analysis and PDEs. Specifically, he studies problems related to the long-time behaviour of nonlinear dispersive equations, as well as Strichartz and restriction estimates. An added complication is that he considers such problems on nontrivial underlying manifolds.

He obtained his PhD in 2011 at the Institute of Applied Physics and Computational Mathematics in Beijing and has since then also spent a year at both the Australian National University and Stanford University.

Welcome Junyong!