An interesting class of feedback matrices, also explored by Jot [217], is that of triangular matrices. A basic fact from linear algebra is that triangular matrices (either lower or upper triangular) have all of their eigenvalues along the diagonal.4.13 For example, the matrix
is lower triangular, and its eigenvalues are
It is important to note that not all triangular matrices are lossless. For example, consider
It has two eigenvalues equal to 1, which looks lossless, but a quick calculation shows that there is only one eigenvector,
which is clearly not lossless.
One way to avoid ``coupled repeated poles'' of this nature is to use
non-repeating eigenvalues. Another is to convert
to Jordan
canonical form by means of a similarity transformation, zero any
off-diagonal elements, and transform back [332].