Can all real diagonalizable matrices with real eigenvalues be diagonalized by a symmetric matrix? That is, if $A$ is a real diagonalizable matrix with real eigenvalues, can we write
$$ A = S D S^{-1} $$
for some real symmetric matrix $S$ and some real diagonal matrix $D$?
Best Answer
Let us consider the more general setting in which $A$ is an $n\times n$ diagonalisable matrix over a field $\mathbb F$. By assumption, $A$ admits a diagonalisation $A=VD_1V^{-1}$. We are asking whether there exist a symmetric matrix $S$ and a diagonal matrix $D$ over $\mathbb F$ such that $VD_1V^{-1} = SDS^{-1}$. Since the two sides have the same spectrum, $D_1=PDP^T$ for some permutation $P$. Thus the equation can be rewritten as $$ (VP)D(VP)^{-1} = SDS^{-1}.\tag{1} $$ We now consider two cases:
The remaining case where $n\ge3$ and $\mathbb F$ has fewer than $n$ elements is more intricate. Since $A$ must contain some repeated eigenvalues, the above counterexample does not apply.