Strings in abstract root systems

Abstract

Let $\Phi$ be a subset of the simple roots of a (possibly non-reduced) abstract root system $\Sigma$, and let $\lambda \in \Sigma$. We define the $\Phi$-string of $\lambda$ as the set of elements in $\Sigma \cup \{0\}$ of the form $\lambda + \sum_{\alpha \in \Phi} n_\alpha \alpha$, where $n_\alpha$ is an integer for each $\alpha \in \Phi$. This notion can be regarded as some sort of generalization of the classical notion of $\alpha$-string, where $\alpha \in \Sigma$.