This is a subject I've been working on for a very long time now, but still did not manage to fully understand the interesting properties of this matrix $\mathbf{A}$.


First, let's define two matrices:


* $\mathbf{N}$ is the following matrix:
\begin{equation}
\mathbf{N}=\begin{bmatrix} \mathbf{I}\_n & \mathbf{0}\_n \\ \mathbf{0}\_n & \mathbf{P}^{-1}\begin{bmatrix}1 & && \\ & \ddots && \\ & & 1& \\ &&& -1 \end{bmatrix}\mathbf{P} \end{bmatrix}\in\mathbb{R}^{2n\times2n}
\end{equation}
where $\mathbf{P}\in\mathbb{R}^{n\times n}$ is any invertible matrix.
* with $\omega\_i>0$ and $t>0$, the block-diagonal matrix:
\begin{equation}
\mathbf{S}(t)=\begin{bmatrix} \begin{bmatrix} \cos(\omega\_1t) & \\& \ddots & \\ & & \cos(\omega\_n t) \end{bmatrix} & \begin{bmatrix} \dfrac{\sin(\omega\_1t)}{\omega\_1} & \\& \ddots & \\ & & \dfrac{\sin(\omega\_nt)}{\omega\_n} \end{bmatrix} \\
\begin{bmatrix} -\omega\_1 \sin(\omega\_1t) & \\& \ddots & \\ & & -\omega\_n\sin(\omega\_n t) \end{bmatrix} & \begin{bmatrix} \cos(\omega\_1t) & \\& \ddots & \\ & & \cos(\omega\_n t) \end{bmatrix}\end{bmatrix}\in\mathbb{R}^{2n\times2n}
\end{equation}


The eigenvalues of $\mathbf N$ are of course 1 (multiplicity $2n-1$) and $-1$ (multiplicity $1$). The eigenvalues of $\mathbf{S}(t)$, which is an exponential matrix, are the $n$ couples of the complex conjugates $(\exp(i\omega\_jt),\overline{\exp(i\omega\_jt)})$.


Now, we can define $\forall t>0$, $\mathbf{A}(t)=\mathbf N\mathbf S(t)$. We know that the product of the eigenvalues of $\mathbf{A}(t)$ is the product of those of $\mathbf{N}$ and $\mathbf S(t)$, i.e. $-1$.


I observe an interesting property but can't prove where it stems from:



> 
> * $1$ and $-1$ are eigenvalues of $\mathbf{A}(t)$ ($\forall t$);
> * $1$ and $-1$ are $\color{red}{\text{not}}$ eigenvalues of $\mathbf{A}(t\_2)\mathbf{A}(t)$ ($\forall t,t\_2$, except maybe for specific values of $\mathbf P$ and $\omega\_k$);
> * $1$ and $-1$ are eigenvalues of $\mathbf{A}(t\_3)\mathbf{A}(t\_2)\mathbf{A}(t)$ ($\forall t,t\_2,t\_3$);
> * $1$ and $-1$ are $\color{red}{\text{not}}$ eigenvalues of $\mathbf{A}(t\_4)\mathbf{A}(t\_3)\mathbf{A}(t\_2)\mathbf{A}(t)$ ($\forall t,t\_2,t\_3,t\_4$, except maybe for specific values of $\mathbf P$ and $\omega\_k$);
> * $\dots$
> 
> 
> 


I managed to prove $1$ and $-1$ are eigenvalues of $\mathbf{A}(t)$ by considering $\mathbf{S}(t)\pm\operatorname{diag}(1,\dots,1,-1,\dots,1)$, calculating its kernel, and building the appropriate vectors (without having to calculate them *explicitly*).


Also, I understand that the product of the eigenvalues of $\mathbf{A}(t\_2)\mathbf{A}(t)$ is 1, while that of $\mathbf{A}(t\_3)\mathbf{A}(t\_2)\mathbf{A}(t)$ is -1, but that does not prove anything.




---


**Questions**


1) Any suggestion to prove the framed observation would be very welcome: why are apparently 1 and -1 eigenvalues of $\prod\_{i=1}^m A(t\_i)$ if and only if $m$ is odd? 


2) Also, I have the impression that there exists a powerful mathematical framework to study these matrices, but I can't figure out which one, as not being a mathematician; Lie algebra because $\mathbf S(t)$ is an exponential? Galois groups because the eigenvalues are complex conjugate? Zariski topology because @loup blanc [mentioned it](https://math.stackexchange.com/a/971763/36578) (see end of answer)?




---


A simple `Mathematica` file to reproduce the results is available [here](http://www.filedropper.com/eigenvaluesstandalone). Just play with the arguments of `calculateEigenvals` to change the dimension $n$ or/and the exponent $m$ (to prove: 1,-1 eigenvalues iff $m$ is odd).


Note that I have already asked the question on [math.SX](https://math.stackexchange.com/questions/1168635/why-are-1-and-1-eigenvalues-of-this-matrix) but have not got any answer.



Why are 1 and -1 eigenvalues of this matrix?
First, one should conjugate all matrices by
$$
\begin{pmatrix}
\operatorname{diag}(\omega\_1,\dots,\omega\_n) & 0 \\ 0 & 1 
\end{pmatrix}
$$
as this converts $S(t)$ to a rotation matrix while leaving the reflection $N$ unchanged.


The matrix $P^{-1} \operatorname{diag}(1,\dots,1,-1) P$ has a line as its -1 eigenspace and a hyperplane as its +1 eigenspace. By a limiting argument we may take $P$ to be in general position, so that the -1 eigenspace is not contained in any coordinate hyperplane and the +1 eigenspace does not contain any coordinate line. Then after applying a further conjugation by a diagonal matrix in the lower $n \times n$ block, we can arrange for the -1 and +1 eigenspaces to be orthogonal, without affecting $A(t)$.


After all these conjugations, $S(t)$ is now orthogonal and orientation preserving, while $N$ is orthogonal and orientation reversing. Then it is now clear that the product of any odd number of the $A(t)$ will be orientation-reversing orthogonal matrices. Such matrices have spectrum on the unit circle, symmetric with respect to conjugation, and multiplying to -1, hence must have an odd number of eigenvalues at +1 and also at -1. (If instead one multiplies an even number of $A(t)$ together, one obtains an even number of eigenvalues at -1 and at +1, but usually one expects to get zero eigenvalues at either.)


