Abstract. We study the fth term in the asymptotic expansionof the heat operator trace for a partial dierential operator of Laplace type on a compact Riemannian manifold with Dirichlet or Neumann boundary conditions. (n m)=2 a n (F;D;B) where the a n (F;D;B) are locally computable; see [6] for details. We computed a n for n 4 in [3]; we h a v e c hanged notation slightly from that paper. In this paper, we compute a 5 if the boundary is totally geodesic. If the boundary is not totally geodesic, the second fundamental form L enters and the combinatorial complexity becomes formidable. In this setting, we also obtain a result by restricting attention 1991 Mathematics Subject Classication. Primary 58G25.

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Abstract.Results in the spectral theory of differential operators, and recent results on conformally covariant differential operators and on sharp inequalities, are combined in a study of functional determinants of natural differential operators. The setting is that of compact Riemannian manifolds. We concentrate especially on the conformally flat case, and obtain formulas in dimensions 2, 4, and 6 for the functional determinants of operators which are well behaved under conformai change of metric. The two-dimensional formulas are due to Polyakov, and the four-dimensional formulas to Branson and Orsted; the method is sufficiently streamlined here that we are able to present the sixdimensional case for the first time. In particular, we solve the extremal problems for the functional determinants of the conformai Laplacian and of the square of the Dirac operator on S2 , and in the standard conformai classes on S4 and S6 . The S2 results are due to Onofri, and the S4 results to Branson, Chang, and Yang; the S6 results are presented for the first time here. Recent results of Graham, Jenne, Mason, and Sparling on conformally covariant differential operators, and of Beckner on sharp Sobolev and Moser-Trudinger type inequalities, are used in an essential way, as are a computation of the spectra of intertwining operators for the complementary series of SOo(m + 1, 1), and the precise dependence of all computations on the dimension. In the process of solving the extremal problem on S6 , we are forced to derive a new and delicate conformally covariant sharp inequality, essentially a covariant form of the Sobolev embedding L2(S6) «-» L3(S6) for section spaces of trace free symmetric two-tensors.

On conformal manifolds of even dimension n ≥ 4 we construct a family of new conformally invariant differential complexes, each containing one coboundary operator of order greater than 1. Each bundle in each of these complexes appears either in the de Rham complex or in its dual (which is a different complex in the non-orientable case). Each of the new complexes is elliptic in case the conformal structure has Riemannian signature. We also construct gauge companion operators which (for differential forms of order k ≤ n/2) complete the exterior derivative to a conformally invariant and (in the case of Riemannian signature) elliptically coercive system. These (operator,gauge) pairs are used to define finite dimensional conformally stable form subspaces which are are candidates for spaces of conformal harmonics. This generalises the n/2-form and 0-form cases, in which the harmonics are given by conformally invariant systems. These constructions are based on a family of operators on closed forms which generalise in a natural way Branson's Qcurvature. We give a universal construction of these new operators and show that they yield new conformally invariant global pairings between differential form bundles. Finally we give a geometric construction of a family of conformally invariant differential operators between densityvalued differential form bundles and develop their properties (including their ellipticity type in the case of definite conformal signature). The construction is based on the ambient metric of Fefferman and Graham, and its relationship to the tractor bundles for the Cartan normal conformal connection. For each form order, our derivation yields an operator of every even order in odd dimensions, and even order operators up to order n in even dimension n. In the case of unweighted (or true) forms as domain, these operators are the natural form analogues of the critical order conformal Laplacian of Graham et al., and are key ingredients in the new differential complexes mentioned above.

Abstract.Results in the spectral theory of differential operators, and recent results on conformally covariant differential operators and on sharp inequalities, are combined in a study of functional determinants of natural differential operators. The setting is that of compact Riemannian manifolds. We concentrate especially on the conformally flat case, and obtain formulas in dimensions 2, 4, and 6 for the functional determinants of operators which are well behaved under conformai change of metric. The two-dimensional formulas are due to Polyakov, and the four-dimensional formulas to Branson and Orsted; the method is sufficiently streamlined here that we are able to present the sixdimensional case for the first time. In particular, we solve the extremal problems for the functional determinants of the conformai Laplacian and of the square of the Dirac operator on S2 , and in the standard conformai classes on S4 and S6 . The S2 results are due to Onofri, and the S4 results to Branson, Chang, and Yang; the S6 results are presented for the first time here. Recent results of Graham, Jenne, Mason, and Sparling on conformally covariant differential operators, and of Beckner on sharp Sobolev and Moser-Trudinger type inequalities, are used in an essential way, as are a computation of the spectra of intertwining operators for the complementary series of SOo(m + 1, 1), and the precise dependence of all computations on the dimension. In the process of solving the extremal problem on S6 , we are forced to derive a new and delicate conformally covariant sharp inequality, essentially a covariant form of the Sobolev embedding L2(S6) «-» L3(S6) for section spaces of trace free symmetric two-tensors.