Monday, 8 September 2008

dg.differential geometry - Normal coordinates for a manifold with volume form

I'm hoping that the following are true. In fact, they are probably easy, but I'm not seeing the answers immediately.



Let M be a smooth m-dimensional manifold with chosen positive smooth density mu, i.e. a chosen (adjectives) volume form. (A density on M is a section of a certain trivial line bundle. In local coordinates, the line bundle is given by the transition maps tildemu=left|detfracpartialtildexpartialxright|mu. When M is oriented, this bundle can be identified with the top exterior power of the cotangent bundle.) Hope 1: Near each point in M there exist local coordinates x:UtomathbbRm so that mu pushes forward to the canonical volume form dx on mathbbRm.



Hope 1 is certainly true for volume forms that arise as top powers of symplectic forms, for example, by always working in Darboux coordinates. If Hope 1 is true, then M has an atlas in which all transition maps are volume-preserving. My second Hope tries to describe these coordinate-changes more carefully.



Let U be a domain in mathbbRm. Recall that a change-of-coordinates tildex(x):UtomathbbRm is oriented-volume-preserving iff fracpartialtildexpartialx is a section of a trivial rmSL(n) bundle on U. An infinitesimal change-of-coordinates is a vector field v on U, thought of as the map xmapstox+epsilonv(x). An infinitesimal change-of-coordinates is necessarily orientation-preserving; it is volume preserving iff fracpartialvpartialx(x) is a section of a trivial mathfraksl(n) bundle on U. Hope 2: The space of oriented-volume-preserving changes-of-coordinates is generated by the infinitesimal volume-preserving changes-of-coordinates, analogous to the way a finite-dimensional connected Lie group is generated by its Lie algebra.



Hope 2 is not particularly well-written, so Hope 2.1 is that someone will clarify the statement. Presumably the most precise statement uses infinite-dimensional Lie groupoids. The point is to show that a certain a priori coordinate-dependent construction in fact depends only on the volume form by showing that the infinitesimal changes of coordinates preserve the construction.



Edit: I have preciseified Hope 2 as this question.

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