The normal symbol on Riemannian manifolds

TL;DR

This paper introduces the normal symbol for pseudodifferential operators on Riemannian manifolds, establishing a bijective correspondence with operators, and provides formulas for compositions, adjoints, and ellipticity.

Contribution

It defines the normal symbol on Riemannian manifolds, linking symbols and operators via integral formulas and extending the concept of ellipticity.

Findings

Pseudodifferential operators can be reconstructed from their normal symbols.

A composition formula for symbols of operators is derived.

A new notion of ellipticity using the normal symbol is proposed.

Abstract

For an arbitrary Riemannian manifold $X$ and Hermitian vector bundles $E$ and $F$ over $X$ we define the notion of the normal symbol of a pseudodifferential operator $P$ from $E$ to $F$. The normal symbol of $P$ is a certain smooth function from the cotangent bundle $T^*X$ to the homomorphism bundle $Hom (E,F)$ and depends on the metric structures resp. the corresponding connections on $X$, $E$ and $F$. It is shown that by a natural integral formula the pseudodifferential operator $P$ can be recovered from its symbol. Thus, modulo smoothing operators resp. smoothing symbols, we receive a linear bijective correspondence between the space of symbols and the space of pseudodifferential operators on $X$. This correspondence comprises a natural transformation between appropriate functors. A formula for the asymptotic expansion of the product symbol of two pseudodifferential operators in terms of the symbols of its factors is given. Furthermore an expression for the symbol of the adjoint is derived. Finally the question of invertibility of pseudodifferential operators is considered. For that we use the normal symbol to establish a new and general notion of elliptic pseudodifferential operators on manifolds.