Hodge star
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Let
be a Riemannian n-manifold with metric
. The Hodge star operator is a linear operator from i-differential forms to (n-i)-differential forms
defined as follows: Let
be a local orthonormal co-frame (i.e., a collection of locally defined 1-forms which are orthonormal with respect to the induced metric on the cotangent space). Then we define
where the plus or minus is chosen so that
is the volume form on
. To define the Hodge star operator for general forms, we simply extend the above definition by linearity.
Example
Give
the standard metric so that
is a coframe. Then the volume form is
. Thus,
and in general