Laplace's equation
In mathematics and physics, Laplace's equation is a second-order partial differential equation named after Pierre-Simon Laplace, who first studied its properties. This is often written as formula_1 or formula_2 where formula_3 is the Laplace operator, formula_4 is the divergence operator (also symbolized "div"), formula_5 is the gradient operator (also symbolized "grad"), and formula_6 is a twice-differentiable real-valued function. The Laplace operator therefore maps a scalar function to another scalar function. If the right-hand side is specified as a given function, formula_7, we have formula_8 This is called Poisson's equation, a  generalization of Laplace's equation. Laplace's equation and Poisson's equation are the simplest examples of elliptic partial differential equations. Laplace's equation is also a special case of the Helmholtz equation. The general theory of solutions to Laplace's equation is known as potential theory. The twice continuously differentiable solutions of Laplace's equation are the harmonic functions, which are important in multiple branches of physics, notably electrostatics, gravitation, and fluid dynamics. In the study of heat conduction, the Laplace equation is the steady-state heat equation. In general, Laplace's equation describes situations of equilibrium, or those  that do not depend explicitly on time. Forms in different coordinate systems. In rectangular coordinates, formula_9 In cylindrical coordinates, formula_10 In spherical coordinates, using the formula_11 convention, formula_12 More generally, in curvilinear coordinates, formula_13 or formula_14 Boundary conditions. The Dirichlet problem for Laplace's equation consists of finding a solution on some domain such that on the boundary of is equal to some given function. Since the Laplace operator appears in the heat equation, one physical interpretation of this problem is as follows: fix the temperature on the boundary of the domain according to the given specification of the boundary  condition. Allow heat to flow until a stationary state is reached in which the temperature at each point on the domain doesn't change anymore. The temperature distribution in the interior will then be given by the solution to the corresponding Dirichlet problem. The Neumann boundary conditions for Laplace's equation specify not the function itself on the boundary of but its normal derivative. Physically, this corresponds to the construction of a potential for a vector field whose effect is known at the boundary of alone. For the example of the heat equation it amounts to prescribing the heat flux  through the boundary. In particular, at an adiabatic boundary, the normal derivative of is zero. Solutions of Laplace's equation are called harmonic functions; they are all analytic within the domain where the equation is satisfied. If any two functions are solutions to Laplace's equation (or any linear homogeneous differential equation), their sum (or any linear combination) is also a solution. This property, called the principle of superposition, is very useful. For example, solutions to complex problems can be constructed by summing simple solutions. In two dimensions. Laplace's equation in two independent variables in rectangular coordinates has the  form formula_15 Analytic functions. The real and imaginary parts of a complex analytic function both satisfy the Laplace equation. That is, if , and if formula_16 then the necessary condition that be analytic is that and be differentiable and that the Cauchy–Riemann equations be satisfied: formula_17 where is the first partial derivative of with respect to . It follows that formula_18 Therefore satisfies the Laplace equation. A similar calculation shows that also satisfies the Laplace equation. Conversely, given a harmonic function, it is the real part of an analytic function, (at least locally). If a trial form is formula_19 then the  Cauchy–Riemann equations will be satisfied if we set formula_20 This relation does not determine , but only its increments: formula_21 The Laplace equation for implies that the integrability condition for is satisfied: formula_22 and thus may be defined by a line integral. The integrability condition and Stokes' theorem implies that the value of the line integral connecting two points is independent of the path. The resulting pair of solutions of the Laplace equation are called conjugate harmonic functions. This construction is only valid locally, or provided that the path does not loop around a singularity. For example, if and are  polar coordinates and formula_23 then a corresponding analytic function is formula_24 However, the angle is single-valued only in a region that does not enclose the origin. The close connection between the Laplace equation and analytic functions implies that any solution of the Laplace equation has derivatives of all orders, and can be expanded in a power series, at least inside a circle that does not enclose a singularity. This is in sharp contrast to solutions of the wave equation, which generally have less regularity. There is an intimate connection between power series and Fourier series. If we expand a  function in a power series inside a circle of radius , this means that formula_25 with suitably defined coefficients whose real and imaginary parts are given by formula_26 Therefore formula_27 which is a Fourier series for . These trigonometric functions can themselves be expanded, using multiple angle formulae. Fluid flow. Let the quantities and be the horizontal and vertical components of the velocity field of a steady incompressible, irrotational flow in two dimensions. The continuity condition for an incompressible flow is that formula_28 and the condition that the flow be irrotational is that formula_29 If we define the differential of a function  by formula_30 then the continuity condition is the integrability condition for this differential: the resulting function is called the stream function because it is constant along flow lines. The first derivatives of are given by formula_31 and the irrotationality condition implies that satisfies the Laplace equation. The harmonic function that is conjugate to is called the velocity potential. The Cauchy–Riemann equations imply that formula_32 Thus every analytic function corresponds to a steady incompressible, irrotational, inviscid fluid flow in the plane. The real part is the velocity potential, and the imaginary part is the stream function. Electrostatics. According to Maxwell 's equations, an electric field in two space dimensions that is independent of time satisfies formula_33 and formula_34 where is the charge density. The first Maxwell equation is the integrability condition for the differential formula_35 so the electric potential may be constructed to satisfy formula_36 The second of Maxwell's equations then implies that formula_37 which is the Poisson equation. The Laplace equation can be used in three-dimensional problems in electrostatics and fluid flow just as in two dimensions. In three dimensions. Fundamental solution. A fundamental solution of Laplace's equation satisfies formula_38 where the Dirac delta function denotes  a unit source concentrated at the point . No function has this property: in fact it is a distribution rather than a function; but it can be thought of as a limit of functions whose integrals over space are unity, and whose support (the region where the function is non-zero) shrinks to a point (see weak solution). It is common to take a different sign convention for this equation than one typically does when defining fundamental solutions. This choice of sign is often convenient to work with because −Δ is a positive operator. The definition of the fundamental solution thus  implies that, if the Laplacian of is integrated over any volume that encloses the source point, then formula_39 The Laplace equation is unchanged under a rotation of coordinates, and hence we can expect that a fundamental solution may be obtained among solutions that only depend upon the distance from the source point. If we choose the volume to be a ball of radius around the source point, then Gauss' divergence theorem implies that formula_40 It follows that formula_41 on a sphere of radius that is centered on the source point, and hence formula_42 Note that, with the opposite sign convention  (used in physics), this is the potential generated by a point particle, for an inverse-square law force, arising in the solution of Poisson equation. A similar argument shows that in two dimensions formula_43 where denotes the natural logarithm. Note that, with the opposite sign convention, this is the potential generated by a pointlike sink (see point particle), which is the solution of the Euler equations in two-dimensional incompressible flow. Green's function. A Green's function is a fundamental solution that also satisfies a suitable condition on the boundary of a volume . For instance, formula_44 may satisfy formula_45  formula_46 Now if is any solution of the Poisson equation in : formula_47 and assumes the boundary values on , then we may apply Green's identity, (a consequence of the divergence theorem) which states that formula_48 The notations "u" and "G" denote normal derivatives on . In view of the conditions satisfied by and , this result simplifies to formula_49 Thus the Green's function describes the influence at of the data and . For the case of the interior of a sphere of radius , the Green's function may be obtained by means of a reflection : the source point at distance from the  center of the sphere is reflected along its radial line to a point "P"' that is at a distance formula_50 Note that if is inside the sphere, then "P′" will be outside the sphere. The Green's function is then given by formula_51 where denotes the distance to the source point and denotes the distance to the reflected point "P"′. A consequence of this expression for the Green's function is the Poisson integral formula. Let , , and be spherical coordinates for the source point . Here denotes the angle with the vertical axis, which is contrary to the usual American mathematical  notation, but agrees with standard European and physical practice. Then the solution of the Laplace equation with Dirichlet boundary values inside the sphere is given by formula_52 where formula_53 is the cosine of the angle between and . A simple consequence of this formula is that if is a harmonic function, then the value of at the center of the sphere is the mean value of its values on the sphere. This mean value property immediately implies that a non-constant harmonic function cannot assume its maximum value at an interior point. Laplace's spherical harmonics. Laplace's equation in  spherical coordinates is: formula_54 Consider the problem of finding solutions of the form . By separation of variables, two differential equations result by imposing Laplace's equation: formula_55 The second equation can be simplified under the assumption that has the form . Applying separation of variables again to the second equation gives way to the pair of differential equations formula_56 formula_57 for some number . A priori, is a complex constant, but because must be a periodic function whose period evenly divides , is necessarily an integer and is a linear combination of the complex exponentials . The solution function is regular at the poles  of the sphere, where . Imposing this regularity in the solution of the second equation at the boundary points of the domain is a Sturm–Liouville problem that forces the parameter to be of the form for some non-negative integer with ; this is also explained below in terms of the orbital angular momentum. Furthermore, a change of variables transforms this equation into the Legendre equation, whose solution is a multiple of the associated Legendre polynomial . Finally, the equation for has solutions of the form ; requiring the solution to be regular throughout forces . Here the solution was assumed to have the  special form . For a given value of , there are independent solutions of this form, one for each integer with . These angular solutions are a product of trigonometric functions, here represented as a complex exponential, and associated Legendre polynomials: formula_58 which fulfill formula_59 Here is called a spherical harmonic function of degree and order , is an associated Legendre polynomial, is a normalization constant, and and represent colatitude and longitude, respectively. In particular, the colatitude , or polar angle, ranges from at the North Pole, to at the Equator, to at the South Pole, and the longitude , or azimuth, may assume all values  with . For a fixed integer , every solution of the eigenvalue problem formula_60 is a linear combination of . In fact, for any such solution, is the expression in spherical coordinates of a homogeneous polynomial that is harmonic (see below), and so counting dimensions shows that there are linearly independent such polynomials. The general solution to Laplace's equation in a ball centered at the origin is a linear combination of the spherical harmonic functions multiplied by the appropriate scale factor , formula_61 where the are constants and the factors are known as solid harmonics. Such an expansion is valid in the ball  formula_62 For formula_63, the solid harmonics with negative powers of formula_64 are chosen instead. In that case, one needs to expand the solution of known regions in Laurent series (about formula_65), instead of Taylor series (about formula_66), to match the terms and find formula_67. Electrostatics. Let formula_68 be the electric field, formula_69 be the electric charge density, and formula_70 be the permittivity of free space. Then Gauss's law for electricity (Maxwell's first equation) in differential form states formula_71 Now, the electric field can be expressed as the negative gradient of the electric potential formula_72, formula_73 if the field  is irrotational, formula_74. The irrotationality of formula_68 is also known as the electrostatic condition. formula_76 formula_77 Plugging this relation into Gauss's law, we obtain Poisson's equation for electricity, formula_78 In the particular case of a source-free region, formula_79 and Poisson's equation reduces to Laplace's equation for the electric potential. If the electrostatic potential formula_72 is specified on the boundary of a region formula_81, then it is uniquely determined. If formula_81 is surrounded by a conducting material with a specified charge density formula_69, and if the total charge formula_84 is known, then formula_72 is also unique . A potential that doesn't satisfy Laplace's equation together with the boundary condition is an invalid electrostatic potential. Gravitation. Let formula_86 be the gravitational field, formula_69 the mass density, and formula_88 the gravitational constant. Then Gauss's law for gravitation in differential form is formula_89 The gravitational field is conservative and can therefore be expressed as the negative gradient of the gravitational potential: formula_90 Using the differential form of Gauss's law of gravitation, we have formula_91 which is Poisson's equation for gravitational fields. In empty space, formula_92 and we have formula_93 which is Laplace's equation for  gravitational fields. In the Schwarzschild metric. S. Persides solved the Laplace equation in Schwarzschild spacetime on hypersurfaces of constant . Using the canonical variables , , the solution is formula_94 where is a spherical harmonic function, and formula_95 Here and are Legendre functions of the first and second kind, respectively, while is the Schwarzschild radius. The parameter is an arbitrary non-negative integer.