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