Can we say that Eigenvalues of symmetric orthogonal matrix must be $+1$ and $-1$?
Since eigenvalues of symmetric matrices are real and eigenvalues of orthogonal matrix have unit modulus. Combining both result eigenvalues of symmetric orthogonal matrices must be $+1$ and $-1$.
Please clarify whether I am correct? Is there any other approach to solve this problem?
Thanks
Best Answer
Yes, you're right. Also note that if $A^\top A=I$ and $A=A^\top$, then $A^2=I$, and now it's immediate that $\pm 1$ are the only possible eigenvalues. (Indeed, applying the spectral theorem, you can now conclude that any such $A$ can only be an orthogonal reflection across some subspace.)