QR Algorithm for a Tridiagonal Matrix
Computing the eigenvalues is O(n**2) arithmetic, but computing the eigenvectors is O(n**3)
Run the QR algorithm on each processor
Save the plane rotations on each processor
Let each processor compute k of the eigenvectors
QR is backward stable, but not forward stable, so sequence of plane rotations may be different
Have one processor run the QR algorithm and broadcast the plane rotations