John's equation

John's equation is an ultrahyperbolic partial differential equation satisfied by the X-ray transform of a function. It is named after Fritz John. Given a function $f:{\mathbb{ℝ}}^n\to \mathbb{ℝ}$ with compact support the X-ray transform is the integral over all lines in ${\mathbb{ℝ}}^n$. We will parameterise the lines by pairs of points $x,y\in {\mathbb{ℝ}}^n$, $x\ne y$ on each line and define $u$ as the ray transform where

$u(x,y)=\underset{-\mathrm{\infty }}{\overset{\mathrm{\infty }}{\int }}f(x+t(y-x))dt.$

Such functions $u$ are characterized by John's equations

$\frac{{\partial }^{2}u}{\partial x_i\partial y_j}-\frac{{\partial }^{2}u}{\partial y_i\partial x_j}=0$

which is proved by Fritz John for dimension three and by Kurusa for higher dimensions.

In three-dimensional x-ray computerized tomography John's equation can be solved to fill in missing data, for example where the data is obtained from a point source traversing a curve, typically a helix.

More generally an ultrahyperbolic partial differential equation (a term coined by Richard Courant) is a second order partial differential equation of the form

$\sum _{i,j=1}^{2n}{a}_{ij}\frac{{\partial }^{2}u}{\partial x_i\partial x_j}+\sum _{i=1}^{2n}{b}_i\frac{\partial u}{\partial x_i}+cu=0$

where $n\ge 2$, such that the quadratic form

$\sum _{i,j=1}^{2n}{a}_{ij}{\xi }_i{\xi }_j$

can be reduced by a linear change of variables to the form

$\sum _{i=1}^n{\xi }_i^2-\sum _{i=n+1}^{2n}{\xi }_i^2.$

It is not possible to arbitrarily specify the value of the solution on a non-characteristic hypersurface. John's paper however does give examples of manifolds on which an arbitrary specification of u can be extended to a solution.

References

• John, Fritz (1938), "The ultrahyperbolic differential equation with four independent variables", Duke Mathematical Journal 4 (2): 300–322, doi:10.1215/S0012-7094-38-00423-5, ISSN 0012-7094, http://projecteuclid.org/euclid.dmj/1077490637 
• Á. Kurusa, A characterization of the Radon transform's range by a system of PDEs, J. Math. Anal. Appl., 161(1991), 218--226. doi:10.1016/0022-247X(91)90371-6
• S K Patch, Consistency conditions upon 3D CT data and the wave equation, Phys. Med. Biol. 47 No 15 (7 August 2002) 2637-2650 doi:10.1088/0031-9155/47/15/306