A note on the singular value decomposition of (skew-)involutory and (skew-)coninvolutory matrices

The singular values $\sigma >1$ of an $n\times n$ involutory matrix $A$ appear in pairs $(\sigma ,\frac{1}{\sigma }),$ while the singular values $\sigma = 1$mayappearinpairs$(1,1)$ or by themselves. The left and right singular vectors of pairs of singular values are closely connected. This link is used to reformulate the singular value decomposition (SVD) of an involutory matrix as an eigendecomposition. This displays an interesting relation between the singular values of an involutory matrix and its eigenvalues. Similar observations hold for the SVD, the singular values and the coneigenvalues of (skew-)coninvolutory matrices.