# Math 7390

Harmonic Analysis
M-W-F: 9:40-10:30AM Lockett 111

 INSTRUCTOR Gestur Olafsson Office 322 Lockett Office Hours Monday 10:40 --11:30 am, Wednesday 1:40-2:30 pm and by request Phone 578-1608 and 225-337-2206 (cell) e-mail olafsson@math.lsu.edu Internet http://math.lsu.edu/~olafsson Text Lecture Notes by R. Fabec and G. Olafsson

The subject proper of harmonic analysis is to decompose functions into ''simpler'' functions, where the meaning of simple depends on the context we are working in. In the theory of differential equations that means to write an arbitrary functions as a superposition of eigenfunctions. If we have a symmetry group acting on the system, then we would like to write an arbitary function as a sum of functions that transforms in a simple and controllable way under the symmetry group. The simplest example is the use of polar coordinates and radial functions for rotation symmetric equations. Sometimes, we are working with general spaces than Rn. Here anlysis meets with Lie groups, geometry, representation theory and harmonic analysis to form abstract harmonic analysis.

The course begins with a short overview of classical Fourier analysis on the torus and Rn. This leads us to topics like:

1. Periodic functions and Fourier series;
2. Convergence of Fourier series;
3. Spaces of functions on Rn. In particular, we will discuss the space of compactly supported functions, functions of compact support and the algebraic structure of those spaces, i.e., convolution.
4. The Fourier transform of rapidly decreasing functions and L2-functions, inversion formula and Plancherel theorems.
5. Introduction to distribution theory and the continuous linear functionals on function spaces. How to differentiate distributions. The Fourier transform of distributions.
6. Application of the Fourier transform to differential equations. In particular we will discuss the heat equation and the wave equation.
7. Hermite functions and polynomials.
8. At the end, we will also discuss some other integral transforms. In partiuclar, we will discuss the continuous wavelet transform, derive a Plancherel formula and an inversion formula.

We can view Rn as a set or as amanifold. But we can also view it as an abelian group. In that sense Rn  is a part of abelian harmonic analysis. The simplest example of  nonabelian harmonic analysis is the Heisenberg group Hn=R2n+1  (with a new group multiplication). The Heisenberg group is also a simple example of a Lie group and of a topological group . There are several other well known examples of topological groups like the group of rotations SO(n) , the group of all invertible matrices GL (n,R). Depending on the time and interest we will at the end discuss some advanced topics related to topological groups.

Representations of topological groups are central in several branches of mathematics: In number theory and the study of authomorphic functions and forms, in geometry as a tool to construct important vector bundles and differential operators, and in the study of Riemannian symmetric spaces. Finally, those are important tools in analysis, in particular analysis on some special homogeneous manifolds like the sphere, Grassmanians, the upper half plane and its generalizations. Representations even shows up in branches of applied mathematics as generalizations of the windowed Fourier transform and wavelets. Several examples of those applications in analysis and geometry will be discussed in the class.

The second part of this class takes plase in the spring 2008, where the weight is on topological groups, homogeneous spaces and representation theory. The instructore will be Mark Davidson.

### References:

• The Lecture Notes: Noncommutative Harmonic Analysis by R. Fabec and G. Olafsson;
• Fourier Analysis by T. W. Koerner;
• Noncommutative Harminic Analysis by Michael E. Taylor;
• Harmonic Analysis on Phase Space by G. Folland

### Homework and exams

There will be the following requirements:
• Set of homeworks every second week
• A midterm exam
• Instead of a final, the students will have to give presentation on a selected topic in harmonic analysis. As an example:
• The Fourier transform and the heat equation;
• The wave equation in two and three dimensional space; Why can we listen to music in three dimensional space and not in a two dimensional space;
• Behavior of Fourier series at points of discontinuity;
• just to name few possibilities.