We shall describe a new proof of the Mordell-Lang conjecture in positive characteristic, in the situation where the variety under scrutiny is a smooth subvariety of an abelian variety. Our proof is based on the theory of semistable sheaves in positive characteristic, in particular on Langer's theorem that the Harder-Narasimhan filtration of sheaves becomes strongly semistable after a finite number of iterations of Frobenius pull-backs. Our proof produces a numerical upper-bound for the degree of the finite morphism from an isotrivial variety appearing in the statement of the Mordell-Lang conjecture. This upper-bound is given in terms of the Frobenius-stabilised slopes of the cotangent bundle of the variety.
In finance, the default time of a counterparty is sometimes modeled as the
first passage time of a credit index process below a barrier. It is
therefore relevant to consider the following question:
If we know the distribution of the default time, can we find a unique
barrier which gives this distribution? This is known as the Inverse
First Passage Time (IFPT) problem in the literature.
We consider a more general `smoothed' version of the inverse first
passage time problem in which the first passage time is replaced by
the first instant that the time spent below the barrier exceeds an
independent exponential random variable. We show that any smooth
distribution results from some unique continuously differentiable
barrier. In current work with B. Ettinger and T. K. Wong, we use PDE
methods to show the uniqueness and existence of solutions to a
discontinuous version of the IFPT problem.
Systemic risk in financial markets occurs when activities that are beneficial to an agent in isolation (e.g. reducing microprudential risk) cause unintended consequences due to collective interactions (usually called macroprudential risk). I will discuss three different mechanisms through which this occurs in financial markets. Contagion can propagate due to the market impact of trading among agents with strongly overlapping portfolios, or due to cascading failures from chains of default caused by networks of interlinked counterparty exposures. A proper understanding of these phenomena must take both dynamics and network effects into account. I will discuss four different examples that illustrate these points. The first is a simple model of the market dynamics induced by Basel-style risk management, which from extremely simple assumptions shows that excessive leverage can give rise to a slowly rising price bubble followed by an abrupt crash with a time period of 10 - 15 years. The model gives rise to a chaotic attractor whose time series closely resembles the Great Moderation and subsequent crisis. We show that alternatives to Basel can provide a better compromise between micro and macro prudential risk. The second example is a model of leveraged value investors that yields clustered volatility and fat-tailed returns similar to those in financial markets. The third example is the DebtRank algorithm, which uses a similar method to PageRank to correctly quantify the way risk propagates through networks of counterparty exposures and can be used as the basis of a systemic risk tax. The fourth example will be work in progress to provide an early warning system for financial stress caused by overlapping portfolios. Finally I will discuss an often neglected source of financial risk due to imbalances in market ecologies.
Pengyu Wei's title: Ranking ForexMaster Players
Abstract:
In this talk I will introduce ForexMaster, a simulated foreign exchange trading platform, and how I rank players on this platform. Different methods are compared. In particular, I use random forest and a carefully chosen feature set, which includes not only traditional performance measures like Sharp ratio, but also estimates from the Plackett-Luce ranking model, which has not been used in the financial modelling yet. I show players selected by this method have satisfactory out-of-sample performance, and the Plackett-Luce model plays an important role.
Alissa Kleinnijenhuis title: Stress Testing the European Banking System: Exposure Risk & Overlapping Portfolio Risk
Abstract:
Current regulatory stress testing, as for example done by the EBA, BoE and the FED, is microprudential, non-systemic. These stress tests do not take into account systemic risk, even though the official aim of the stress test is the "test the resilience of the financial system as a whole, and the individual banks therein, to another crisis".
Two papers are being developed that look at the interconnections between banks. One paper investigates the systemic risk in the European banking system due to interbank exposures, using EBA data. The other paper, looks at the trade-off between individual and systemic risk with overlapping portfolios. The above two "channels of contagion" for systemic risk can be incorporated in stress tests to include systemic components to the traditional non-systemic stress tests.
We describe how p-adic height pairings allow us to find integral points on hyperelliptic curves, in the spirit of Kim's nonabelian Chabauty program. In particular, we discuss how to carry out this ``quadratic Chabauty'' method over quadratic number fields (joint work with Amnon Besser and Steffen Mueller) and present related ideas to find rational points on bielliptic genus 2 curves (joint work with Netan Dogra).
Nonlinear systems of partial differential equations (PDEs) may permit several distinct solutions. The typical current approach to finding distinct solutions is to start Newton's method with many different initial guesses, hoping to find starting points that lie in different basins of attraction. In this talk, we present an infinite-dimensional deflation algorithm for systematically modifying the residual of a nonlinear PDE problem to eliminate known solutions from consideration. This enables the Newton--Kantorovitch iteration to converge to several different solutions, even starting from the same initial guess. The deflated Jacobian is dense, but an efficient preconditioning strategy is devised, and the number of Krylov iterations is observed not to grow as solutions are deflated. The technique is then applied to computing distinct solutions of nonlinear PDEs, tracing bifurcation diagrams, and to computing multiple local minima of PDE-constrained optimisation problems.
Preconditioning is of significant importance in the solution of large dimensional systems of linear equations such as those that arise from the numerical solution of partial differential equation problems. In this talk we will attempt a broad ranging review of preconditioning.