Parabolic cylindrical coordinates

Basic definition

The parabolic cylindrical coordinates (σ, τ, z) are defined in terms of the Cartesian coordinates (x, y, z) by:

:\begin{align}

x &= \sigma \tau \ y &= \frac{1}{2} \left( \tau^2 - \sigma^2 \right) \ z &= z \end{align}

The surfaces of constant σ form confocal parabolic cylinders

: 2 y = \frac{x^2}{\sigma^2} - \sigma^2

that open towards +y, whereas the surfaces of constant τ form confocal parabolic cylinders

: 2 y = -\frac{x^2}{\tau^2} + \tau^2

that open in the opposite direction, i.e., towards −y. The foci of all these parabolic cylinders are located along the line defined by . The radius r has a simple formula as well

: r = \sqrt{x^2 + y^2} = \frac{1}{2} \left( \sigma^2 + \tau^2 \right)

that proves useful in solving the Hamilton–Jacobi equation in parabolic coordinates for the inverse-square central force problem of mechanics; for further details, see the Laplace–Runge–Lenz vector article.

Scale factors

The scale factors for the parabolic cylindrical coordinates σ and τ are:

:\begin{align} h_\sigma &= h_\tau = \sqrt{\sigma^2 + \tau^2} \ h_z &= 1 \end{align}

Differential elements

The infinitesimal element of volume is

:dV = h_\sigma h_\tau h_z d\sigma d\tau dz = ( \sigma^2 + \tau^2 ) d\sigma , d\tau , dz

The differential displacement is given by:

:d\mathbf{l} = \sqrt{\sigma^2 + \tau^2} , d\sigma , \boldsymbol{\hat{\sigma}} + \sqrt{\sigma^2 + \tau^2} , d\tau , \boldsymbol{\hat{\tau}} + dz , \mathbf{\hat{z}}

The differential normal area is given by:

:d\mathbf{S} = \sqrt{\sigma^2 + \tau^2} , d\tau , dz \boldsymbol{\hat{\sigma}} + \sqrt{\sigma^2 + \tau^2} , d\sigma , dz \boldsymbol{\hat{\tau}} + \left(\sigma^2 + \tau^2\right) , d\sigma , d\tau \mathbf{\hat{z}}

Del

Let f be a scalar field. The gradient is given by

:\nabla f = \frac{1}{\sqrt{\sigma^{2} + \tau^{2}}} {\partial f \over \partial \sigma}\boldsymbol{\hat{\sigma}} + \frac{1}{\sqrt{\sigma^{2} + \tau^{2}}} {\partial f \over \partial \tau}\boldsymbol{\hat{\tau}} + {\partial f \over \partial z}\mathbf{\hat{z}}

The Laplacian is given by

:\nabla^2 f = \frac{1}{\sigma^{2} + \tau^{2}} \left(\frac{\partial^{2} f}{\partial \sigma^{2}} + \frac{\partial^{2} f}{\partial \tau^{2}} \right) + \frac{\partial^{2} f}{\partial z^{2}}

Let A be a vector field of the form:

:\mathbf A = A_\sigma \boldsymbol{\hat{\sigma}} + A_\tau \boldsymbol{\hat{\tau}} + A_z \mathbf{\hat{z}}

The divergence is given by

:\nabla \cdot \mathbf A = \frac{1}{\sigma^{2} + \tau^{2}}\left({\partial (\sqrt{\sigma^2+\tau^2} A_\sigma) \over \partial \sigma} + {\partial (\sqrt{\sigma^2+\tau^2} A_\tau) \over \partial \tau}\right) + {\partial A_z \over \partial z}

The curl is given by

:\nabla \times \mathbf A = \left( \frac{1}{\sqrt{\sigma^2 + \tau^2}} \frac{\partial A_z}{\partial \tau}

• \frac{\partial A_\tau}{\partial z} \right) \boldsymbol{\hat{\sigma}}
• \left( \frac{1}{\sqrt{\sigma^2 + \tau^2}} \frac{\partial A_z}{\partial \sigma}
• \frac{\partial A_\sigma}{\partial z} \right) \boldsymbol{\hat{\tau}}

• \frac{1}{\sigma^2 + \tau^2} \left( \frac{\partial \left(\sqrt{\sigma^2 + \tau^2} A_\tau \right)}{\partial \sigma}

• \frac{\partial \left(\sqrt{\sigma^2 + \tau^2} A_\sigma\right)}{\partial \tau} \right) \mathbf{\hat{z}}

Other differential operators can be expressed in the coordinates (σ, τ) by substituting the scale factors into the general formulae found in orthogonal coordinates.

Relationship to other coordinate systems

Relationship to cylindrical coordinates (ρ, φ, z):

:\begin{align} \rho\cos\varphi &= \sigma \tau\ \rho\sin\varphi &= \frac{1}{2} \left( \tau^2 - \sigma^2 \right) \ z &= z \end{align}

Parabolic unit vectors expressed in terms of Cartesian unit vectors:

:\begin{align} \boldsymbol{\hat{\sigma}} &= \frac{\tau \hat{\mathbf x} - \sigma \hat{\mathbf y}}{\sqrt{\tau^2+\sigma^2}} \ \boldsymbol{\hat{\tau}} &= \frac{\sigma \hat{\mathbf x} + \tau \hat{\mathbf y}}{\sqrt{\tau^2+\sigma^2}} \ \mathbf{\hat{z}} &= \mathbf{\hat{z}} \end{align}

Parabolic cylinder harmonics

Since all of the surfaces of constant σ, τ and z are conicoids, Laplace's equation is separable in parabolic cylindrical coordinates. Using the technique of the separation of variables, a separated solution to Laplace's equation may be written:

:V = S(\sigma) T(\tau) Z(z)

and Laplace's equation, divided by V, is written:

:\frac{1}{\sigma^2 + \tau^2} \left[\frac{\ddot{S}}{S} + \frac{\ddot{T}}{T}\right] + \frac{\ddot{Z}}{Z} = 0

Since the Z equation is separate from the rest, we may write

:\frac{\ddot{Z}}{Z}=-m^2

where m is constant. Z(z) has the solution:

:Z_m(z)=A_1,e^{imz}+A_2,e^{-imz}

Substituting −m2 for \ddot{Z} / Z, Laplace's equation may now be written:

:\left[\frac{\ddot{S}}{S} + \frac{\ddot{T}}{T}\right] = m^2 (\sigma^2 + \tau^2)

We may now separate the S and T functions and introduce another constant n2 to obtain:

:\ddot{S} - (m^2\sigma^2 + n^2) S = 0 :\ddot{T} - (m^2\tau^2 - n^2) T = 0

The solutions to these equations are the parabolic cylinder functions

:S_{mn}(\sigma) = A_3 y_1(n^2 / 2m, \sigma \sqrt{2m}) + A_4 y_2(n^2 / 2m, \sigma \sqrt{2m}) :T_{mn}(\tau) = A_5 y_1(n^2 / 2m, i \tau \sqrt{2m}) + A_6 y_2(n^2 / 2m, i \tau \sqrt{2m})

The parabolic cylinder harmonics for (m, n) are now the product of the solutions. The combination will reduce the number of constants and the general solution to Laplace's equation may be written:

:V(\sigma, \tau, z) = \sum_{m, n} A_{mn} S_{mn} T_{mn} Z_m

Applications

The classic applications of parabolic cylindrical coordinates are in solving partial differential equations, e.g., Laplace's equation or the Helmholtz equation, for which such coordinates allow a separation of variables. A typical example would be the electric field surrounding a flat semi-infinite conducting plate.

Bibliography

• Same as Morse & Feshbach (1953), substituting u**k for ξk.