Products of Idempotents in Banach Algebras of Operators
Let $X$ be a Banach space and $\mathcal A$ be the Banach algebra $B(X)$ of bounded (i.e. continuous) linear transformations (to be called operators) on $X$ to itself. Let $\mathcal E$ be the set of idempotents in $\mathcal A$ and $\mathcal S$ be the semigroup generated by $\mathcal E$ under composition as multiplication. If $T\in \mathcal S$ with $0\ne T\ne I_{X}$ then $T$ has a local block representation of the form $\begin{pmatrix} T_1 & T_2 0 & 0 \end{pmatrix}$ on $X=Y\oplus Z$, a topological sum of non-zero closed subspaces $Y$ and $Z$ of $X$, and any $A\in \mathcal A$ has the form $\begin{pmatrix} A_1 & A_2 A_3 & A_4 \end{pmatrix}$ with $T_1,A_1 \in \mathcal B(Y)$, $T_2,A_2\in B(Z,Y), A_3\in B(Y,Z)$, and $A_4 \in \mathcal B(Z)$. The purpose of this paper is to study conditions for $T$ to be in $\mathcal{S}$.