# Laplace operator

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Laplace operator

In mathematics the Laplace operator or Laplacian is a differential operator given by the divergence of the gradient of a function on Euclidean space. It is usually denoted by the symbols ∇·∇, ∇2 or Δ. The Laplacian Δƒ(p) of a function ƒ at a point p, up to a constant depending on the dimension, is the rate at which the average value of ƒ over spheres centered at p, deviates from ƒ(p) as the radius of the sphere grows. In a Cartesian coordinate system, the Laplacian is given by sum of all the (unmixed) second partial derivatives of the function. In other coordinate systems such as cylindrical and spherical coordinates, the Laplacian also has a useful form.

The Laplace operator is named after the French mathematician Pierre-Simon de Laplace (1749–1827), who first applied the operator to the study of celestial mechanics, where the operator gives a constant multiple of the mass density when it is applied to a given gravitational potential. Solutions of the equation Δƒ = 0, now called Laplace's equation, are the so-called harmonic functions, and represent the possible gravitational fields in free space.

It occurs in the differential equations that describe many physical phenomena, such as electric and gravitational potentials, the diffusion equation for heat and fluid flow, wave propagation, and quantum mechanics. The Laplacian represents the flux density of the gradient flow of a function. For instance, the net rate at which a chemical dissolved in a fluid moves toward or away from some point is proportional to the Laplacian of the chemical concentration at that point; expressed symbolically, the resulting equation is the diffusion equation. For these reasons, it is extensively used in the sciences for modelling all kinds of physical phenomena. The Laplacian is the simplest elliptic operator, and is at the core of Hodge theory as well as the results of de Rham cohomology. In image processing and computer vision, the Laplacian operator has been used for various tasks such as blob and edge detection.

## Definition

The Laplace operator is a second order differential operator in the n-dimensional Euclidean space, defined as the divergence (∇·) of the gradient (∇ƒ). Thus if ƒ is a twice-differentiable real-valued function, then the Laplacian of ƒ is defined by $\Delta f = \nabla^2 f = \nabla \cdot \nabla f,$   (1)

Equivalently, the Laplacian of ƒ is the sum of all the unmixed second partial derivatives in the Cartesian coordinates xi: $\Delta f = \sum_{i=1}^n \frac {\partial^2 f}{\partial x^2_i}.$   (2)

As a second-order differential operator, the Laplace operator maps Ck-functions to Ck−2-functions for k ≥ 2. The expression (1) (or equivalently (2)) defines an operator Δ : Ck(Rn) → Ck−2(Rn), or more generally an operator Δ : Ck(Ω) → Ck−2(Ω) for any open set Ω.

## Motivation

### Diffusion

In the physical theory of diffusion, the Laplace operator (via Laplace's equation) arises naturally in the mathematical description of equilibrium. Specifically, if u is the density at equilibrium of some quantity such as a chemical concentration, then the net flux of u through the boundary of any smooth region V is zero, provided there is no source or sink within V: $\int_{\partial V} \nabla u \cdot \mathbf{n}\, dS = 0,$

where n is the outward unit normal to the boundary of V. By the divergence theorem, $\int_V \mathrm{div} \nabla u\, dV = \int_{\partial V} \nabla u\cdot\mathbf{n}\, dS = 0.$

Since this holds for all smooth regions V, it can be shown that this implies $\mathrm{div} \nabla u = \Delta u = 0.$

The left-hand side of this equation is the Laplace operator. The Laplace operator itself has a physical interpretation for non-equilibrium diffusion as the extent to which a point represents a source or sink of chemical concentration, in a sense made precise by the diffusion equation.

### Density associated to a potential

If φ denotes the electrostatic potential associated to a charge distribution q, then the charge distribution itself is given by the Laplacian of φ: $q = \Delta\varphi.\,$

(1)

This is a consequence of Gauss's law. Indeed, if V is any smooth region, then by Gauss's law the flux of the electrostatic field E is equal to the charge enclosed (in appropriate units): $\int_{\partial V} \mathbf{E}\cdot \mathbf{n} = \int_{\partial V} \nabla\varphi\cdot \mathbf{n} = \int_V q\,dV,$

where the first equality uses the fact that the electrostatic field is the gradient of the electrostatic potential. The divergence theorem now gives $\int_V \Delta\varphi\,dV = \int_V q\, dV,$

and since this holds for all regions V, (1) follows.

The same approach implies that the Laplacian of the gravitational potential is the mass distribution. Often the charge (or mass) distribution are given, and the associated potential is unknown. Finding the potential function subject to suitable boundary conditions is equivalent to solving Poisson's equation.

### Energy minimization

Another motivation for the Laplacian appearing in physics is that solutions to Δf = 0 in a region U are functions that make the Dirichlet energy functional stationary: $E(f) = \frac{1}{2} \int_U \Vert \nabla f \Vert^2 \mathrm{d}x.$

To see this, suppose $f\colon U\to \mathbb{R}$ is a function, and $u\colon U\to \mathbb{R}$ is a function that vanishes on the boundary of U. Then $\frac{d}{d\varepsilon}\Big|_{\varepsilon = 0} E(f+\varepsilon u) = \int_U \nabla f \cdot \nabla u \, \mathrm{d} x = -\int_U u \Delta f \mathrm{d} x$

where the last equality follows using Green's first identity. This calculation shows that if Δf = 0, then E is stationary around f. Conversely, if E is stationary around f, then Δf = 0 by the fundamental lemma of calculus of variations.

## Coordinate expressions

### Two dimensions

The Laplace operator in two dimensions is given by $\Delta f = \frac{\partial^2f}{\partial x^2} + \frac{\partial^2 f}{\partial y^2}$

where x and y are the standard Cartesian coordinates of the xy-plane.

In polar coordinates, \begin{align} \Delta f &= {1 \over r} {\partial \over \partial r} \left( r {\partial f \over \partial r} \right) + {1 \over r^2} {\partial^2 f \over \partial \theta^2}\\ &= {1 \over r} {\partial f \over \partial r} + {\partial^2 f \over \partial r^2} + {1 \over r^2} {\partial^2 f \over \partial \theta^2} . \end{align}

### Three dimensions

In three dimensions, it is common to work with the Laplacian in a variety of different coordinate systems.

In Cartesian coordinates, $\Delta f = \frac{\partial^2 f}{\partial x^2} + \frac{\partial^2 f}{\partial y^2} + \frac{\partial^2 f}{\partial z^2}.$

In cylindrical coordinates, $\Delta f = {1 \over \rho} {\partial \over \partial \rho} \left( \rho {\partial f \over \partial \rho} \right) + {1 \over \rho^2} {\partial^2 f \over \partial \theta^2} + {\partial^2 f \over \partial z^2 }.$

In spherical coordinates: $\Delta f = {1 \over r^2} {\partial \over \partial r} \left( r^2 {\partial f \over \partial r} \right) + {1 \over r^2 \sin \varphi} {\partial \over \partial \varphi} \left( \sin \varphi {\partial f \over \partial \varphi} \right) + {1 \over r^2 \sin^2 \varphi} {\partial^2 f \over \partial \theta^2}.$

(here θ represents the azimuthal angle and φ the zenith angle or co-latitude).

In general curvilinear coordinates (ξ123): $\nabla^2 = \nabla \xi^m \cdot \nabla \xi^n {\partial^2 \over \partial \xi^m \partial \xi^n} + \nabla^2 \xi^m {\partial \over \partial \xi^m },$

where summation over the repeated indices is implied.

### N dimensions

In spherical coordinates in N dimensions, with the parametrization x = rθ ∈ RN with r representing a positive real radius and θ an element of the unit sphere SN−1, $\Delta f = \frac{\partial^2 f}{\partial r^2} + \frac{N-1}{r} \frac{\partial f}{\partial r} + \frac{1}{r^2} \Delta_{S^{N-1}} f$

where $\Delta_{S^{N-1}}$ is the Laplace–Beltrami operator on the (N−1)-sphere, known as the spherical Laplacian. The two radial terms can be equivalently rewritten as $\frac{1}{r^{N-1}} \frac{\partial}{\partial r} \Bigl(r^{N-1} \frac{\partial f}{\partial r} \Bigr).$

As a consequence, the spherical Laplacian of a function defined on SN−1 ⊂ RN can be computed as the ordinary Laplacian of the function extended to RN\{0} so that it is constant along rays, i.e., homogeneous of degree zero.

## Spectral theory

The spectrum of the Laplace operator consists of all eigenvalues λ for which there is a corresponding eigenfunction ƒ with

− Δf = λf.

If Ω is a bounded domain in Rn then the eigenfunctions of the Laplacian are an orthonormal basis for the Hilbert space L2(Ω). This result essentially follows from the spectral theorem on compact self-adjoint operators, applied to the inverse of the Laplacian (which is compact, by the Poincaré inequality and Kondrakov embedding theorem). It can also be shown that the eigenfunctions are infinitely differentiable functions. More generally, these results hold for the Laplace–Beltrami operator on any compact Riemannian manifold with boundary, or indeed for the Dirichlet eigenvalue problem of any elliptic operator with smooth coefficients on a bounded domain. When Ω is the n-sphere, the eigenfunctions of the Laplacian are the well-known spherical harmonics.

## Generalizations

### Laplace–Beltrami operator

The Laplacian also can be generalized to an elliptic operator called the Laplace–Beltrami operator defined on a Riemannian manifold. The d'Alembert operator generalizes to a hyperbolic operator on pseudo-Riemannian manifolds. The Laplace–Beltrami operator, when applied to a function, is the trace of the function's Hessian: $\Delta f = \mathrm{tr}(H(f))\,\!$

where the trace is taken with respect to the inverse of the metric tensor. The Laplace–Beltrami operator also can be generalized to an operator (also called the Laplace–Beltrami operator) which operates on tensor fields, by a similar formula.

Another generalization of the Laplace operator that is available on pseudo-Riemannian manifolds uses the exterior derivative, in terms of which the Laplacian is expressed as $\Delta f = d^* d f\,$

Here d is the codifferential, which can also be expressed using the Hodge dual. More generally, the Laplacian is defined on differential forms α by $\Delta \alpha = d^* d\alpha + dd^*\alpha.\,$

This is known as the Laplace–de Rham operator, which is related Laplace–Beltrami operator by the Weitzenböck identity.

### D'Alembertian

The Laplacian can be generalized in certain ways to non-Euclidean spaces, where it may be elliptic, hyperbolic, or ultrahyperbolic.

In the Minkowski space the Laplace–Beltrami operator becomes the d'Alembert operator or d'Alembertian: $\square = \frac {1}{c^2}{\partial^2 \over \partial t^2 } - {\partial^2 \over \partial x^2 } - {\partial^2 \over \partial y^2 } - {\partial^2 \over \partial z^2 }.$

It is the generalisation of the Laplace operator in the sense that it is the differential operator which is invariant under the isometry group of the underlying space and it reduces to the Laplace operator if restricted to time independent functions. Note that the overall sign of the metric here is chosen such that the spatial parts of the operator admit a negative sign, which is the usual convention in high energy particle physics. The D'Alembert operator is also known as the wave operator, because it is the differential operator appearing in the wave equations and it is also part of the Klein–Gordon equation, which reduces to the wave equation in the massless case. The additional factor of c in the metric is needed in physics if space and time are measured in different units; a similar factor would be required if, for example, the x direction were measured in meters while the y direction were measured in centimeters. Indeed, theoretical physicists usually work in units such that c=1 in order to simplify the equation.

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