Laplace operators in differential geometry

In differential geometry there are a number of second-order, linear, elliptic differential operators bearing the name Laplacian. This article provides an overview of some of them.

1 Connection Laplacian
2 Hodge Laplacian
3 Bochner Laplacian
4 Lichnerowicz Laplacian
5 Conformal Laplacian
6 See also
7 References

Connection Laplacian

The connection Laplacian is a differential operator acting on the various tensor bundles of a manifold, defined in terms of a Riemannian- or pseudo-Riemannian metric. When applied to functions (i.e, tensors of rank 0), the connection Laplacian is often called the Laplace–Beltrami operator. It is defined as the trace of the second covariant derivative:

$\Delta T= -\text{tr}\;\nabla^2 T,$

where T is any tensor, $\nabla$ is the Levi-Civita connection associated to the metric, and the trace is taken with respect to the metric. Recall that the second covariant derivative of T is defined as

$\nabla^2_{X,Y} T = -\left(\nabla_X \nabla_Y T - \nabla_{\nabla_X Y} T\right).$

Note that with this definition, the connection Laplacian has negative spectrum. On functions, it agrees with the operator given as the divergence of the gradient.

Hodge Laplacian

The Hodge Laplacian, also known as the Laplace–de Rham operator, is differential operator on acting on differential forms. (Abstractly, it is a second order operator on each exterior power of the cotangent bundle.) This operator is defined on any manifold equipped with a Riemannian- or pseudo-Riemannian metric.

$\Delta= \mathrm{d}\delta%2B\delta\mathrm{d} = (\mathrm{d}%2B\delta)^2,\;$

where d is the exterior derivative or differential and δ is the codifferential. The Hodge Laplacian on a compact manifold has nonnegative spectrum.

The connection Laplacian may also be taken to act on differential forms by restricting it to act on skew-symmetric tensors. The connection Laplacian differs from the Hodge Laplacian by means of a Weitzenböck identity.

Bochner Laplacian

The Bochner Laplacian is defined differently from the connection Laplacian, but the two will turn out to differ only by a sign, whenever the former is defined. Let M be a compact, oriented manifold equipped with a metric. Let E be a vector bundle over M equipped a fiber metric and a compatible connection, $\nabla$ . This connection gives rise to a differential operator

$\nabla:\Gamma(E)\rightarrow \Gamma(T^*M\otimes E)$

where $\Gamma(E)$ denotes smooth sections of E, and T^*M is the cotangent bundle of M. It is possible to take the $L^2$ -adjoint of $\nabla$ , giving a differential operator

$\nabla^*:\Gamma(T^*M\otimes E)\rightarrow \Gamma(E).$

The Bochner Laplacian is given by

$\Delta=\nabla^*\nabla$

which is a second order operator acting on sections of the vector bundle E. Note that the connection Laplacian and Bochner Laplacian differ only by a sign:

$\nabla^* \nabla = - \text{tr}\, \nabla^2$

Lichnerowicz Laplacian

The Lichnerowicz Laplacian^[1] is defined on symmetric tensors by taking $\nabla�: \Gamma(\operatorname{Sym}^k(TM))\to \Gamma(\operatorname{Sym}^{k%2B1}(TM))$ to be the symmetrized covariant derivative. The Lichnerowicz Laplacian is then defined by $\Delta_L = \nabla^*\nabla$ , where $\nabla^*$ is the formal adjoint. The Lichnerowicz Laplacian differs from the usual tensor Laplacian by a Weitzenbock formula involving the Riemann curvature tensor, and has natural applications in the study of Ricci flow and the prescribed Ricci curvature problem.

Conformal Laplacian

On a Riemannian manifold, one can define the conformal Laplacian as an operator on smooth functions; it differs from the Laplace–Beltrami operator by a term involving the scalar curvature of the underlying metric. In dimension n ≥ 3, the conformal Laplacian, denoted L, acts on a smooth function u by

$Lu = -4\frac{n-1}{n-2} \Delta u %2B Ru,$

where Δ is the Laplace-Beltrami operator (of negative spectrum), and R is the scalar curvature. This operator often makes an appearance when studying how the scalar curvature behaves under a conformal change of a Riemannian metric. If n ≥ 3 and g is a metric and u is a smooth, positive function, then the conformal metric

$\tilde g = u^{4/(n-2)} g \,$

has scalar curvature given by

$\tilde R = u^{-(n%2B2)/(n-2)} L u. \,$

References

^ Chow, Bennett; Lu, Peng; Ni, Lei (2006), Hamilton's Ricci flow, Graduate Studies in Mathematics, 77, Providence, R.I.: American Mathematical Society, ISBN 978-0-8218-4231-7; 978-0-8218-4231-7, MR 2274812