Abstract. We give an introductory account of functional determinants of elliptic operators on manifolds and Polyakov-type formulas for their infinitesimal and finite conformal variations. We relate this to extremal problems and to the Q-curvature on even-dimensional conformal manifolds. The exposition is self-contained, in the sense of giving references sufficient to allow the reader to work through all details. Originally we had planned to publish the lecture notes of the instructional lecturers. This paper was submitted by Professor Branson for that purpose. In fact this was the only paper we had received by the deadline. Hence we decided to cancel the plan for a proceedings volume. Just after that period, Thomas Branson unexpectedly passed away. We held the paper without knowing what we could do with it. When the editors of this proceedings volume invited me to submit an article, I realised that this would be an ideal place for Professor Thomas Branson's paper. I immediately submitted the paper to editors of the current proceedings. I would like to take this opportunity to express my sincere appreciation to the editors for their help.Xingwang Xu (National University of Singapore) E-mail: matxuxw@nus.edu.sg
The functional determinantIn order to get a feel for the spectral theory of natural differential operators on compact manifolds, recall the idea of Fourier series, where one attempts to expand complex functions on the unit circle S 1 in C in the formThe trigonometric series is an expansion in real eigenfunctions of the Laplacian ∆ = −d 2 /dθ 2 (the eigenvalue being j 2 ). The exponential series is an expansion in eigenfunctions of the operator −id/dθ, which is a square root of the Laplacian; the eigenvalue is k.