Involutions of $\ell ^2$ and $s$ with unique fixed points

Abstract:

Let $\sigma _{\ell ^2}$ and $\sigma _{\Bbb R^{\infty }}$ be the linear involutions of $\ell ^2$ and $\mathbb {R}^\infty$, respectively, given by the formula $x\to -x$. We prove that although $\ell ^2$ and $\Bbb R^{\infty }$ are homeomorphic, $\sigma _{\ell ^2}$ is not topologically conjugate to $\sigma _{\Bbb R^{\infty }}$. We proceed to examine the implications of this and give characterizations of the involutions that are conjugate to $\sigma _{\ell ^2}$ and to $\sigma _{\Bbb R^{\infty }}$. We show that the linear involution $x\to -x$ of a separable, infinite-dimensional Fréchet space $E$ is topologically conjugate to $\sigma _{\ell ^2}$ if and only if $E$ contains an infinite-dimensional Banach subspace and otherwise is linearly conjugate to $\sigma _{\Bbb R^{\infty }}$.