Exotic Cluster Structures on $S{L}_n$ with Belavin-Drinfeld Data of Minimal Size, I. The Structure

Using the notion of compatibility between Poisson brackets and cluster structures in the coordinate rings of simple Lie groups, Gekhtman Shapiro and Vainshtein conjectured a correspondence between the two. Poisson Lie groups are classified by the Belavin-Drinfeld classification of solutions to the classical Yang Baxter equation. For any non trivial Belavin-Drinfeld data of minimal size for $S{L}_n$, we give an algorithm for constructing an initial seed $\mathrm{\Sigma }$ in $\mathcal{O}(S{L}_n)$. The cluster structure $\mathcal{C}=\mathcal{C}(\mathrm{\Sigma })$ is then proved to be compatible with the Poisson bracket associated with that Belavin-Drinfeld data, and the seed $\mathrm{\Sigma }$ is locally regular. This is the first of two papers, and the second one proves the rest of the conjecture: the upper cluster algebra ${\bar{\mathcal{A}}}_{\mathbb{C}}(\mathcal{C})$ is naturally isomorphic to $\mathcal{O}(S{L}_n)$, and the correspondence of Belavin-Drinfeld classes and cluster structures is one to one.