In mathematics, the **Helmholtz equation** is the eigenvalue problem for the Laplace operator. It corresponds to the linear partial differential equation

where ∇

The equation is named after Hermann von Helmholtz, who studied it in 1860.^{[1]}

The Helmholtz equation often arises in the study of physical problems involving partial differential equations (PDEs) in both space and time. The Helmholtz equation, which represents a **time-independent** form of the wave equation, results from applying the technique of separation of variables to reduce the complexity of the analysis.

For example, consider the wave equation

Separation of variables begins by assuming that the wave function *u*(**r**, *t*) is in fact separable:

Substituting this form into the wave equation and then simplifying, we obtain the following equation:

Notice that the expression on the left side depends only on **r**, whereas the right expression depends only on t. As a result, this equation is valid in the general case if and only if both sides of the equation are equal to the same constant value. This argument is key in the technique of solving linear partial differential equations by separation of variables. From this observation, we obtain two equations, one for *A*(**r**), the other for *T*(*t*):

where we have chosen, without loss of generality, the expression −*k*^{2} for the value of the constant. (It is equally valid to use any constant k as the separation constant; −*k*^{2} is chosen only for convenience in the resulting solutions.)

Rearranging the first equation, we obtain the Helmholtz equation:

Likewise, after making the substitution *ω* = *kc*, where k is the wave number, and ω is the angular frequency (assuming a monochromatic field), the second equation becomes

We now have Helmholtz's equation for the spatial variable **r** and a second-order ordinary differential equation in time. The solution in time will be a linear combination of sine and cosine functions, whose exact form is determined by initial conditions, while the form of the solution in space will depend on the boundary conditions. Alternatively, integral transforms, such as the Laplace or Fourier transform, are often used to transform a hyperbolic PDE into a form of the Helmholtz equation.

Because of its relationship to the wave equation, the Helmholtz equation arises in problems in such areas of physics as the study of electromagnetic radiation, seismology, and acoustics.

The solution to the spatial Helmholtz equation:

can be obtained for simple geometries using separation of variables.

The two-dimensional analogue of the vibrating string is the vibrating membrane, with the edges clamped to be motionless. The Helmholtz equation was solved for many basic shapes in the 19th century: the rectangular membrane by Siméon Denis Poisson in 1829, the equilateral triangle by Gabriel Lamé in 1852, and the circular membrane by Alfred Clebsch in 1862. The elliptical drumhead was studied by Émile Mathieu, leading to Mathieu's differential equation.

If the edges of a shape are straight line segments, then a solution is integrable or knowable in closed-form only if it is expressible as a finite linear combination of plane waves that satisfy the boundary conditions (zero at the boundary, i.e., membrane clamped).

If the domain is a circle of radius a, then it is appropriate to introduce polar coordinates r and θ. The Helmholtz equation takes the form

We may impose the boundary condition that A vanishes if *r* = *a*; thus

the method of separation of variables leads to trial solutions of the form

where Θ must be periodic of period 2

It follows from the periodicity condition that

and that n must be an integer. The radial component R has the form

where the Bessel function

and

The general solution A then takes the form of a generalized Fourier series of terms involving products of *J _{n}*(

In spherical coordinates, the solution is:

This solution arises from the spatial solution of the wave equation and diffusion equation. Here *j _{ℓ}*(

Writing **r**_{0} = (*x*, *y*, *z*) function *A*(*r*_{0}) has asymptotics

where function f is called scattering amplitude and *u*_{0}(*r*_{0}) is the value of A at each boundary point *r*_{0}.

Given a 2-dimensional plane where A is known, the solution to the Helmholtz equation is given by:^{[2]}

where

- is the solution at the 2-dimensional plane,

As z approaches zero, all contributions from the integral vanish except for r=0. Thus up to a numerical factor, which can be verified to be 1 by transforming the integral to polar coordinates .

This solution is important in diffraction theory, e.g. in deriving Fresnel diffraction.

Further information: Slowly varying envelope approximation |

In the paraxial approximation of the Helmholtz equation,^{[3]} the complex amplitude A is expressed as

where u represents the complex-valued amplitude which modulates the sinusoidal plane wave represented by the exponential factor. Then under a suitable assumption, u approximately solves

where is the transverse part of the Laplacian.

This equation has important applications in the science of optics, where it provides solutions that describe the propagation of electromagnetic waves (light) in the form of either paraboloidal waves or Gaussian beams. Most lasers emit beams that take this form.

The assumption under which the paraxial approximation is valid is that the z derivative of the amplitude function u is a slowly varying function of z:

This condition is equivalent to saying that the angle θ between the wave vector **k** and the optical axis z is small: *θ* ≪ 1.

The paraxial form of the Helmholtz equation is found by substituting the above-stated expression for the complex amplitude into the general form of the Helmholtz equation as follows:

Expansion and cancellation yields the following:

Because of the paraxial inequality stated above, the ∂^{2}*u*/∂*z*^{2} term is neglected in comparison with the *k*·∂*u*/∂*z* term. This yields the paraxial Helmholtz equation. Substituting *u*(**r**) = *A*(**r**) *e*^{−ikz} then gives the paraxial equation for the original complex amplitude A:

The Fresnel diffraction integral is an exact solution to the paraxial Helmholtz equation.^{[4]}

The **inhomogeneous Helmholtz equation** is the equation

where

In order to solve this equation uniquely, one needs to specify a boundary condition at infinity, which is typically the Sommerfeld radiation condition

in spatial dimensions, for all angles (i.e. any value of ). Here where are the coordinates of the vector .

With this condition, the solution to the inhomogeneous Helmholtz equation is

(notice this integral is actually over a finite region, since f has compact support). Here, G is the Green's function of this equation, that is, the solution to the inhomogeneous Helmholtz equation with *f* equaling the Dirac delta function, so G satisfies

The expression for the Green's function depends on the dimension n of the space. One has

for

for

for

Finally, for general n,

where and .^{[5]}