On the structure of the set of algebraic elements in a Banach algebra and their liftings

Abstract

We generalize earlier results about connected components of idempotents in Banach algebras, due to B. Sz\H{o}kefalvi Nagy, Y. Kato, S. Maeda, Z. V. Kovarik, J. Zem\'anek, J. Esterle. Let $A$ be a unital complex Banach algebra, and $p(\lambda) = \prod\limits_{i = 1}^n (\lambda - \lambda_i)$ a polynomial over $\Bbb C$, with all roots distinct. Let $E_p(A) := \{a \in A \mid p(a) = 0\}$. Then all connected components of $E_p(A)$ are pathwise connected (locally pathwise connected) via each of the following three types of paths: 1)~similarity via a finite product of exponential functions (via an exponential function); 2)~a polynomial path (a cubic polynomial path); 3)~a polygonal path (a polygonal path consisting of $n$ segments). If $A$ is a $C^*$-algebra, $\lambda_i \in \Bbb R$, let $S_p(A):= \{a\in A \mid a = a^*$, $p(a) = 0\}$. Then all connected components of $S_p(A)$ are pathwise connected (locally pathwise connected), via a path of the form $e^{-ic_mt}\dots e^{-ic_1t} ae^{ic_1t}\dots e^{ic_mt}$, where $c_i = c_i^*$, and $t \in [0, 1]$ (of the form $e^{-ict} ae^{ict}$, where $c = c^*$, and $t \in [0,1]$). For (self-adjoint) idempotents we have by these old papers that the distance of different connected components of them is at least~$1$. For $E_p(A)$, $S_p(A)$ we pose the problem if the distance of different connected components is at least $\min \bigl\{|\lambda_i - \lambda_j| \mid 1 \leq i,j \leq n, \ i \neq j\bigr\}$. For the case of $S_p(A)$, we give a positive lower bound for these distances, that depends on $\lambda_1, \dots, \lambda_n$. We show that several local and global lifting theorems for analytic families of idempotents, along analytic families of surjective Banach algebra homomorphisms, from our recent paper with B. Aupetit and M. Mbekhta, have analogues for elements of $E_p(A)$ and $S_p(A)$.