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volume element (Definition)

If $ M$ is an $ n$ dimensional manifold, then a differential $ n$ form that is never zero is called a volume element or a volume form. Usually one volume form is associated with the manifold. The volume element is sometimes denoted by $ dV,$ $ \omega$ or $ \operatorname{vol}_n.$ If the manifold is a Riemannian manifold with metric $ g,$ then the natural volume form is defined in local coordinates $ x^1 \ldots x^n$ by

$\displaystyle dV := \sqrt{\lvert g \rvert} dx^1 ~ \wedge \ldots \wedge ~dx^n .$    

It is not hard to show that a manifold has a volume form if and only if it is orientable.

If the manifold is $ {\mathbb{R}}^n,$ then the usual volume element $ dV = dx^1~ \wedge ~ dx^2 ~ \wedge \ldots \wedge ~dx^n$ is called the Euclidean volume element or Euclidean volume form. In this context, $ {\mathbb{C}}^n$ is usually treated as $ {\mathbb{R}}^{2n}$ unless stated otherwise.

When $ n=2$, then the form is frequently called the area element or area form and frequently denoted by $ dA$. Furthermore, when the manifold is a submanifold of $ {\mathbb{R}}^3$, then many authors will refer to a surface area element or surface area form.

When the context is measure theoretic, this form is sometimes called a volume measure, area measure, etc...

Bibliography

1
Michael Spivak. Calculus on Manifolds, W.A. Benjamin, Inc., 1965.
2
William M. Boothby. An Introduction to Differentiable Manifolds and Riemannian Geometry, Academic Press, San Diego, California, 2003.



"volume element" is owned by jirka.
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Other names:  volume form, volume measure
Also defines:  area element, area form, area measure, Euclidean volume element, Euclidean volume form, euclidean volume measure, surface area measure, surface area element, surface area form
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Cross-references: measure, submanifold, orientable, local coordinates, Riemannian manifold, manifold
There are 17 references to this entry.

This is version 2 of volume element, born on 2007-12-12, modified 2007-12-12.
Object id is 10122, canonical name is VolumeElement.
Accessed 2964 times total.

Classification:
AMS MSC53-00 (Differential geometry :: General reference works )
 58A10 (Global analysis, analysis on manifolds :: General theory of differentiable manifolds :: Differential forms)

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