Solving large-scale continuous-time algebraic Riccati equations is a significant challenge in various control theory applications. 

This work demonstrates that when the matrix coefficients of the equation are quasiseparable, the solution also exhibits numerical quasiseparability. This property enables us to develop two efficient Riccati solvers. The first solver is applicable to the general quasiseparable case, while the second is tailored to the particular case of banded coefficients. Numerical experiments confirm the effectiveness of the proposed algorithms on both synthetic examples and case studies from the control of partial differential equations and agent-based models. 

For over 150 years, the study of phase transitions—such as water freezing into ice or magnets losing their magnetism—has been a cornerstone of statistical physics. In this talk, we explore the critical behavior of two-dimensional percolation models, which use random graphs to model the behavior of porous media. At the critical point, remarkable symmetries and emergent properties arise, providing precise insights into the nature of these systems and enriching our understanding of phase transitions. The presentation is designed to be accessible and does not assume any prior background in percolation theory.

About the Speaker: 

Hugo Duminil-Copin is is a French mathematician recognised for his groundbreaking work in probability theory and mathematical physics. He was appointed full professor at the University of Geneva in 2014 and since 2016 has also been a permanent professor at the Institut des Hautes Études Scientifiques (IHES) in France. In 2022 he was awarded the Fields Medal, the highest distinction in mathematics.

Inversion problems, such as full waveform inversion (FWI), based on wave propagation, are computationally costly optimization processes used in many applications, ranging from seismic imaging to brain tomography. In most of these uses, high-order methods are required for both accuracy and computational efficiency. Within finite element methods (FEM), using high(er)-order can provide accuracy and the usage of flexible meshes. However, FEM are rarely employed in connection with unstructured simplicial meshes because of the computational cost and complexity of code implementation. They are used frequently with quadrilateral or hexahedral spectral finite elements, but the mesh adaptivity on those elements has not yet been fully explored. In this work, we address these challenges by developing software that leverages accurate higher-order mass-lumped simplicial elements with a mesh-adaption parameter, allowing us to take advantage of the computational efficiency of newer mass-lumped simplicial elements together with waveform-adapted meshes and the accuracy of higher-order function spaces. We also calculate these mesh-related parameters and develop software for high-order spectral element methods, allowing mesh flexibility. We will also discuss future developments. The open-source code was implemented using the Firedrake framework and the Unified Form Language (UFL), a mathematical-based domain specific language, allowing flexibility in a wide range of wave-based problems.