Laplace operator

Laplacian

The differential operator in defined by the formula (1)

(here are coordinates in ), as well as some generalizations of it. The Laplace operator (1) is the simplest elliptic differential operator of the second order. The Laplace operator plays an important role in mathematical analysis, mathematical physics and geometry (see, for example, Laplace equation; Laplace–Beltrami equation; Harmonic function; Harmonic form). Let be an -dimensional Riemannian manifold with metric (2)

let be the matrix inverse to the matrix and let . Then the Laplace operator (or Laplace–Beltrami operator) on with the Riemannian metric (2) has the form (3)

where are local coordinates on . (The operator (1) differs in sign from the Laplace operator on with the standard Euclidean metric .)

A generalization of the operator (3) is the Laplace operator on differential forms (cf. also Differential form). Namely, in the space of exterior differential forms on the Laplace operator has the form (4)

where is the operator of exterior differentiation of a form and is the operator formally adjoint to , defined by means of the following inner product on smooth forms with compact support: (5)

where is the Hodge star operator induced by the metric (2) taking a -form into an -form. In (5) the forms and are assumed to be real; on complex forms one must use the Hermitian extension of the inner product (5). The restriction of the operator (4) to -forms (that is, functions) is specified by (3). On -forms with an arbitrary integer the Laplace operator in local coordinates can be written in the form    Here and are the covariant derivatives with respect to (cf. Covariant derivative), is the curvature tensor and is the Ricci tensor.

Suppose one is given an arbitrary elliptic complex (6)

where the are real or complex vector bundles on and the are their spaces of smooth sections. Introducing a Hermitian metric in each vector bundle and also specifying the volume element on in an arbitrary way, one can define a Hermitian inner product in the space of smooth sections of with compact support. Then operators formally adjoint to the operator are defined. The Laplace operator (4) is then constructed on each space by formula (3). If for the complex (6) one takes the de Rham complex, then for a natural choice of the metric on the -forms and the volume element induced by the metric (2), one obtains for the Laplace operator of the de Rham complex the Laplace operator on forms, described above.

On a complex manifold , together with the de Rham complex there are also the elliptic complexes (7) (8)

where is the space of smooth forms of type on . Introducing a Hermitian structure in the tangent bundle on , one can construct the Laplace operator (4) of the de Rham complex and the Laplace operators of the complexes (7) and (8):  Each of these operators takes the space into itself. If is a Kähler manifold and the Hermitian structure on is induced by the Kähler metric, then An important fact, which determines the role of the Laplace operator of an elliptic complex, is the existence in the case of a compact manifold of the orthogonal Weyl decomposition: (9)

In this decomposition , where is the Laplace operator of the complex (6), so that is the space of "harmonic" sections of (in the case of the de Rham complex, this is the space of all harmonic forms of degree ). The direct sum of the first two terms on the right-hand side of (9) is equal to , and the direct sum of the last two terms coincides with . In particular, the decomposition (9) gives an isomorphism between the cohomology space of the complex (6) in the term and the space of harmonic sections .