Monday, 15 October 2007

dg.differential geometry - Commutator of Lie derivative and codifferential?

Let (M,g)(M,g) be some smooth, Riemannian manifold. Let dd be the exterior derivative and deltadelta the codifferential on forms. For a smooth vector field XX, let LXLX be the Lie derivative associated to XX. We know from Cartan formula that LX=diotaX+iotaXdLX=diotaX+iotaXd where iotaXiotaX is the interior derivative associated to the vector field XX. So it is well-known that LXLX and dd commute: for any arbitrary form omegaomega, we have that LXdomega=dLXomegaLXdomega=dLXomega.



This is, of course, not true for codifferentials. In general [LX,delta]neq0[LX,delta]neq0. For certain cases the answer is well known: if XX is a Killing vector (LXg=0LXg=0) then since it leaves the metric structure in variant, it commutes with the Hodge star operator, and so LXLX commutes with deltadelta. Another useful case is when XX is conformally Killing with constant conformal factor (LXg=kgLXg=kg with dk=0dk=0). In this case conformality implies that the commutator [LX,]=kalpha[LX,]=kalpha where alphaalpha is some numerical power depending on the rank of the form it is acting on (I think... correct me if I am wrong), so we have that [LX,delta]proptodelta[LX,delta]proptodelta.



So my question is: "Is there a general nice formula for the commutator [LX,delta][LX,delta]?" If it is written down somewhere, a reference will be helpful. (In the Riemannian case, by working with suitable symmetrisations of metric connection one can get a fairly ugly answer by doing something like deltaomegaproptomathoptrg1nablaomegadeltaomegaproptomathoptrg1nablaomega and use that the commutators [LX,g1][LX,g1] and [LX,nabla][LX,nabla] are fairly well known [the latter giving a second-order deformation tensor measuring affine-Killingness]. But this formula is the same for the divergence of arbitrary covariant tensors. I am wondering if there is a better formula for forms in particular.)



A simple explanation of why what I am asking is idiotic would also be welcome.

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