It would be convenient if every real matrix were orthogonally similar to a diagonal matrix,
but unfortunately, it is only the symmetric matrices that have this property. In problems
involving similarity, say similarity to an upper triangular matrix, factorization of the characteristic
polynomial is always a stumbling block and so any result must carry along the
necessary assumptions regarding it. It has been proved that there is no “quadratic formula”
type method for solving polynomial equations of degree five and larger, and so we can feel
sure that this factorization must be assumed separately. Is there a best result that can be
stated, with reasonable assumptions, regarding similarity? An answer will soon appear