What are the eigenvalues of an (orthogonal) projection operator that projects vectors onto some hyperplane passing through the origin?
Well, for vectors $v$ orthogonal to this hyperplane, the projection kills the vector, so that $Pv = 0$, which implies $0$ is an eigenvalue.
What are the rest of the eigenvalues? As $P$ is norm-preserving, $|\lambda_i| = 1$, that I am certain of. But is the eigenvalue exactly $1$? Anymore eigenvalues? Or the eigenvalues are simply $0$ and $1$?
For reflection onto this hyperplane, any vector on this hyperplane is its own reflection, so $Rv = v$, which shows that $1$ is an eigenvalue. For the rest of the eigenvalues, it's easy to see the symmetry from the reflected vector, so that the other eigenvalue must be $-1$, since $Rv = -v = (-1)v$ for $v$ not on the hyperplane.
Any ideas are welcome.