Classification of linkage systems
A linkage diagram is obtained from the Carter diagram $\Gamma$ by adding an extra root $\gamma$, so that the resulting subset of roots is linearly independent. With every linkage diagram we associate the linkage label vector $\gamma^{\nabla}$, similar to Dynkin labels. The linkage diagrams connected under the action of the group $W^{\vee}_{S}$ constitute the the linkage system $\mathscr{L}(\Gamma)$. For any simply-laced Carter diagram, the system $\mathscr{L}(\Gamma)$ is constructed. To obtain linkage diagrams $\theta^{\nabla}$, we use an easily verifiable criterion: $\mathscr{B}^{\vee}_{\Gamma}(\theta^{\nabla}) < 2$, where $\mathscr{B}^{\vee}_{\Gamma}$ is the inverse quadratic form associated with $\Gamma$. A Dynkin diagram $\Gamma'$ such that rank($\Gamma'$) = rank($\Gamma$) + 1 and any $\Gamma$-associated root subset $S$ lies in $\varPhi(\Gamma')$, is said to be the Dynkin extension. The linkage system $\mathscr{L}(\Gamma)$ is the union of $\Gamma_i$-components $\mathscr{L}_{\Gamma_i}(\Gamma)$ taken for all Dynkin extensions of $\Gamma <_D \Gamma_i$. The subset $\varPhi(S)$ of roots of $\varPhi(\Gamma')$, linearly dependent on roots of $S$ is said to be a partial root system. The size of $\mathscr{L}_{\Gamma'}(\Gamma)$ is estimated as follows: $|\mathscr{L}_{\Gamma'}(\Gamma)| \leq |\varPhi(\Gamma')| - |\varPhi(S)|$. Carter diagrams $E_l$ and $E_l(a_i)$ (resp. $D_l$ and $D_l(a_k)$) are said to be covalent. For any pair {$\Gamma, \widetilde\Gamma$} of covalent Carter diagrams, where $\Gamma$ is the Dynkin diagram, we explicitly construct the invertible linear map $M : \mathcal{P} \longrightarrow \mathcal{R}$, where $\mathcal{R}$ (resp. $\mathcal{P}$) is the root system (resp. partial root system) corresponding to $\Gamma$ (resp. $\widetilde\Gamma$). In particular, we have $|\mathscr{L}(\widetilde\Gamma)| = |\mathscr{L}(\Gamma)|$.