The study of Harmonic Analysis provides the foundation for the theory of partial differential equations, several complex variables, and harmonic analysis related to semisimple Lie groups and symmetric spaces. I am interested in four topics in harmonic analysis: (1) singular integral operators; (2) multiplier operators; (3) weighted inequalities for operators; (4) the convergence of the Fourier series in higher dimensional Euclidean spaces. In control theory, I study the stability of nonlinear systems. Results are general and include time variant parameters. While the theorems do not generally provide necessary conditions, they are relatively simple to apply and do not require the development of a Liapunov function. Some of these theorems have been applied to study the spread of tuberculosis. Currently, I am interested in the existence, uniqueness and regularity of solutions to the Navier-Stokes equations.