Past exam of the mathematics course of the University of Cambridge / 2024 / iii / Paper 102 / 3 / a / ii

Write a positive nonsimple root asIf $(\alpha ,\beta )\le 0$ for every simple root with $n_{\beta }>0$, thenwhich is impossible. Hence $(\alpha ,\beta )>0$ for some simple $\beta$. The root-string property then gives $\alpha -\beta \in \Phi$, and its simple-root coefficients remain nonnegative. This is the simple-root subtraction lemma.

Induct on the height $\sum n_{\beta }$. Applying the induction hypothesis to $\alpha -\beta$ and appending $\beta$ writesso that every partial sum is a root.

Finally let $\alpha$ be simple and let $\gamma \ne \alpha$ be positive. In the simple-root expansion ofall coefficients except possibly that of $\alpha$ are unchanged, and at least one of those unchanged coefficients is positive. Since a root has coefficients all of one sign, the image cannot be negative. Thus $s_{\alpha }$ permutes ${\Phi }^{+}\setminus \{\alpha \}$, as stated by action of a simple reflection on positive roots.