Boundary Algorithms for Thompson Groups

We construct a deterministic Farey-boundary algorithm converting a cyclic prefix table for an element of $\mathsf{T}$ into a word in addressed boundary flips. The reduction uses rational boundary pairs and cyclic order, with a marked edge determining the residual modular correction. Its unmarked stage uses at most twice the minimum number of diagonal exchanges. For input size $s$, the complete output has $O(s)$ factors, encoded length $O(s^2)$, and running time $O(s^3)$. Adjoining shuffles extends the construction to $\mathsf{V}$ with polynomial bounds. We also give explicit boundary generating sets and their local flip–shuffle relations. Changes of generators in the Lochak–Schneps and Bleak–Quick presentations yield a two-flip presentation of $\mathsf{T}$ and a presentation of $\mathsf{V}$ with two flips, one shuffle, and nine relations.