Published online by Cambridge University Press: 04 January 2013
We prove that the flat product metric on ${D}^{n} \times {S}^{1} $ is scattering rigid where
${D}^{n} $ is the unit ball in
${ \mathbb{R} }^{n} $ and
$n\geq 2$. The scattering data (loosely speaking) of a Riemannian manifold with boundary is the map
$S: {U}^{+ } \partial M\rightarrow {U}^{- } \partial M$ from unit vectors
$V$ at the boundary that point inward to unit vectors at the boundary that point outwards. The map (where defined) takes
$V$ to
${ \gamma }_{V}^{\prime } ({T}_{0} )$ where
${\gamma }_{V} $ is the unit speed geodesic determined by
$V$ and
${T}_{0} $ is the first positive value of
$t$ (when it exists) such that
${\gamma }_{V} (t)$ again lies in the boundary. We show that any other Riemannian manifold
$(M, \partial M, g)$ with boundary
$\partial M$ isometric to
$\partial ({D}^{n} \times {S}^{1} )$ and with the same scattering data must be isometric to
${D}^{n} \times {S}^{1} $. This is the first scattering rigidity result for a manifold that has a trapped geodesic. The main issue is to show that the unit vectors tangent to trapped geodesics in
$(M, \partial M, g)$ have measure zero in the unit tangent bundle.