Density on a manifold

In general, there does not exist a natural concept of a "volume" for a parallelotope generated by vectors v₁, ..., v_n in a n-dimensional vector space V. However, if one wishes to define a function μ : V × ... × V → R that assigns a volume for any such parallelotope, it should satisfy the following properties:

• If any of the vectors v_k is multiplied by λ ∈ R, the volume should be multiplied by |λ|.
• If any linear combination of the vectors v₁, ..., v_j−1, v_j+1, ..., v_n is added to the vector v_j, the volume should stay invariant.

These conditions are equivalent to the statement that μ is given by a translation-invariant measure on V, and they can be rephrased as

${\displaystyle \mu (Av_{1},\ldots ,Av_{n})=\left|\det A\right|\mu (v_{1},\ldots ,v_{n}),\quad A\in \operatorname {GL} (V).}$

Any such mapping μ : V × ... × V → R is called a density on the vector space V. Note that if (v₁, ..., v_n) is any basis for V, then fixing μ(v₁, ..., v_n) will fix μ entirely; it follows that the set Vol(V) of all densities on V forms a one-dimensional vector space. Any n-form ω on V defines a density |ω| on V by

${\displaystyle |\omega |(v_{1},\ldots ,v_{n}):=|\omega (v_{1},\ldots ,v_{n})|.}$

Formally, the s-density bundle Vol^s(M) of a differentiable manifold M is obtained by an associated bundle construction, intertwining the one-dimensional group representation

${\displaystyle \rho (A)=\left|\det A\right|^{-s},\quad A\in \operatorname {GL} (n)}$

of the general linear group with the frame bundle of M.

The resulting line bundle is known as the bundle of s-densities, and is denoted by

${\displaystyle \left|\Lambda \right|_{M}^{s}=\left|\Lambda \right|^{s}(TM).}$

A 1-density is also referred to simply as a density.

More generally, the associated bundle construction also allows densities to be constructed from any vector bundle E on M.

In detail, if (U_α,φ_α) is an atlas of coordinate charts on M, then there is associated a local trivialization of ${\displaystyle \left|\Lambda \right|_{M}^{s}}$

${\displaystyle t_{\alpha }:\left|\Lambda \right|_{M}^{s}|_{U_{\alpha }}\to \phi _{\alpha }(U_{\alpha })\times \mathbb {R} }$

subordinate to the open cover U_α such that the associated GL(1)-cocycle satisfies

${\displaystyle t_{\alpha \beta }=\left|\det(d\phi _{\alpha }\circ d\phi _{\beta }^{-1})\right|^{-s}.}$

Densities play a significant role in the theory of integration on manifolds. Indeed, the definition of a density is motivated by how a measure dx changes under a change of coordinates (Folland 1999, Section 11.4, pp. 361-362).

Given a 1-density ƒ supported in a coordinate chart U_α, the integral is defined by

${\displaystyle \int _{U_{\alpha }}f=\int _{\phi _{\alpha }(U_{\alpha })}t_{\alpha }\circ f\circ \phi _{\alpha }^{-1}d\mu }$

where the latter integral is with respect to the Lebesgue measure on Rⁿ. The transformation law for 1-densities together with the Jacobian change of variables ensures compatibility on the overlaps of different coordinate charts, and so the integral of a general compactly supported 1-density can be defined by a partition of unity argument. Thus 1-densities are a generalization of the notion of a volume form that does not necessarily require the manifold to be oriented or even orientable. One can more generally develop a general theory of Radon measures as distributional sections of ${\displaystyle |\Lambda |_{M}^{1}}$ using the Riesz-Markov-Kakutani representation theorem.

The set of 1/p-densities such that ${\displaystyle |\phi |_{p}=\left(\int |\phi |^{p}\right)^{1/p}<\infty }$ is a normed linear space whose completion ${\displaystyle L^{p}(M)}$ is called the intrinsic L^p space of M.

In some areas, particularly conformal geometry, a different weighting convention is used: the bundle of s-densities is instead associated with the character

${\displaystyle \rho (A)=\left|\det A\right|^{-s/n}.}$

With this convention, for instance, one integrates n-densities (rather than 1-densities). Also in these conventions, a conformal metric is identified with a tensor density of weight 2.

• The dual vector bundle of ${\displaystyle |\Lambda |_{M}^{s}}$ is ${\displaystyle |\Lambda |_{M}^{-s}}$.
• Tensor densities are sections of the tensor product of a density bundle with a tensor bundle.

• Berline, Nicole; Getzler, Ezra; Vergne, Michèle (2004), Heat Kernels and Dirac Operators, Berlin, New York: Springer-Verlag, ISBN 978-3-540-20062-8.
• Folland, Gerald B. (1999), Real Analysis: Modern Techniques and Their Applications (Second ed.), ISBN 978-0-471-31716-6, provides a brief discussion of densities in the last section.{{citation}}: CS1 maint: postscript (link)
• Nicolaescu, Liviu I. (1996), Lectures on the geometry of manifolds, River Edge, NJ: World Scientific Publishing Co. Inc., ISBN 978-981-02-2836-1, MR 1435504
• Lee, John M (2003), Introduction to Smooth Manifolds, Springer-Verlag