First Integrals of Homogeneous Vector Fields and the Eigenmirror Problem of Geometric Optics
The eigenmirror problem asks: ``When does the reflection of a surface in a curved mirror appear undistorted to an observer?'' We call such a surface an {\em eigensurface} and the corresponding mirror an {\em eigenmirror}. The data for an eigenmirror problem consists of a homogeneous transformation ${\bf H}:\mathbb{R}^3 \to \mathbb{R}^3$ that encodes what it means for two observers to see a surface in the ``same way.'' A solution to this problem is a differentiable 2-manifold that (1) satisfies a first-order partial differential equation called the {\bf anti-eikonal equation}, and (2) satisfies certain side inequalities that ensure that a ray reflecting off the mirror behaves in a physically meaningful way. Although these side inequalities initially seem like an ad hoc global restriction, we show that under reasonable conditions, an integral curve of the characteristic flow of the anti-eikonal equation may not intersect the boundary of an eigenmirror. Thus, in those cases, the eigenmirror is invariant under the characteristic flow. We give several examples exhibiting our results.