Most General Lossless Feedback Matrices
As shown in §C.15.3, an FDN feedback matrix
is
lossless if and only if its eigenvalues have modulus 1 and its
eigenvectors are linearly independent.
A unitary matrix is any (complex) matrix that is inverted
by its own (conjugate) transpose:






All unitary (and orthogonal) matrices have unit-modulus eigenvalues and linearly independent eigenvectors. As a result, when used as a feedback matrix in an FDN, the resulting FDN will be lossless (until the delay-line damping filters are inserted, as discussed in §3.7.4 below).
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Triangular Feedback Matrices
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Householder Reflections