# Mathematics

## On Fourth Order PDEs Modelling Electrostatic Micro-Electronical Systems

Micro-ElectroMechanical Systems (MEMS) and Nano-ElectroMechanical Systems (NEMS) are now a well established sector of contemporary technology. A key component of such systems is the simple idealized electrostatic device consisting of a thin and deformable plate that is held fixed along its boundary , where is a bounded domain in The plate, which lies below another parallel rigid grounded plate (say at level ) has its upper surface coated with a negligibly thin metallic conducting film, in such a way that if a voltage l is applied to the conducting film, it deflects towards the top plate, and if the applied voltage is increased beyond a certain critical value , it then proceeds to touch the grounded plate. The steady-state is then lost, and we have a snap-through at a finite time creating the so-called pull-in instability. A proposed model for the deflection is given by the evolution equation

Now unlike the model involving only the second order Laplacian (i.e., ), very little is known about this equation. We shall explain how, besides the above practical considerations, the model is an extremely rich source of interesting mathematical phenomena.

## Law of Large Number and Central Limit Theorem under Uncertainty, the related New Itô's Calculus and Applications to Risk Measures

Let where is a sequence of independent and identically distributed (i.i.d.) of random variables with . According to the classical law of large number (LLN), the sum converges strongly to . Moreover, the well-known central limit theorem (CLT) tells us that, with and , for each bounded and continuous function we have with .

These two fundamentally important results are widely used in probability, statistics, data analysis as well as in many practical situation such as financial pricing and risk controls. They provide a strong argument to explain why in practice normal distributions are so widely used. But a serious problem is that the i.i.d. condition is very difficult to be satisfied in practice for the most real-time processes for which the classical trials and samplings becomes impossible and the uncertainty of probabilities and/or distributions cannot be neglected.

In this talk we present a systematical generalization of the above LLN and CLT. Instead of fixing a probability measure P, we only assume that there exists a uncertain subset of probability measures . In this case a robust way to calculate the expectation of a financial loss is its upper expectation: where is the expectation under the probability . The corresponding distribution uncertainty of is given by , . Our main assumptions are:

- The distributions of are within an abstract subset of distributions , called the distribution uncertainty of , with and .
- Any realization of does not change the distributional uncertainty of (a new type of `independence' ).

Our new LLN is: for each linear growth continuous function we have

Namely, the distribution uncertainty of is, approximately, .

In particular, if , then converges strongly to 0. In this case, if we assume furthermore that and , . Then we have the following generalization of the CLT:

Here stands for a distribution uncertainty subset and its the corresponding upper expectation. The number can be calculated by defining which solves the following PDE , with

An interesting situation is when is a convex function, with . But if is a concave function, then the above has to be replaced by . This coincidence can be used to explain a well-known puzzle: many practitioners, particularly in finance, use normal distributions with `dirty' data, and often with successes. In fact, this is also a high risky operation if the reasoning is not fully understood. If , then which is a classical normal distribution. The method of the proof is very different from the classical one and a very deep regularity estimate of fully nonlinear PDE plays a crucial role.

A type of combination of LLN and CLT which converges in law to a more general -distributions have been obtained. We also present our systematical research on the continuous-time counterpart of the above `G-normal distribution', called G-Brownian motion and the corresponding stochastic calculus of Itô's type as well as its applications.

## Regular Permutation Groups and Cayley Graphs

## PIMS/UBC Math Department Grad Student/Postdoc Job Forum

## Math Mania

## PIMS Board Meeting - Fall 2011

The PIMS 2011 Fall board meeting was held at the University of Saskatchewan. In addition to the board meeting, board members toured the Canadian Light Source facility.