Monodromy-Matrix Description of Extremal Multi-centered Black Holes

We study solution-generating techniques based on the Breitenlohner--Maison linear system for extremal, stationary biaxisymmetric black hole solutions in five-dimensional $U(1)^3$ supergravity. Focusing on multi-center configurations over a Gibbons--Hawking base, we analyze both BPS and almost-BPS solutions, including rotating single-center black holes and two-center black rings. After dimensional reduction to three dimensions, the system is described by a coset sigma model with target space $SO(4,4)/[SO(2,2)\times SO(2,2)]$, where solutions are encoded in coset and monodromy matrices. For Bena--Warner BPS solutions, we construct the coset and monodromy matrices and show that they admit an exponential representation governed by nilpotent elements. Although the monodromy matrices generically exhibit double poles, they can be factorized explicitly using the nilpotent algebra of $\mathfrak{so}(4,4)$, reconstructing the solutions. We extend this to almost-BPS solutions and derive the corresponding matrices. While the single-center case exhibits commuting residues, the two-center black ring leads to a more intricate structure with a third-order pole, which disappears when regularity is imposed. Finally, we analyze the extremal limits of the Rasheed--Larsen solution, where the fast-rotating branch is governed by idempotent elements. We also construct an explicit $SO(4,4)$ duality transformation relating the slowly-rotating branch to a single-center almost-BPS solution. These results will provide the BM formalism as a unified framework for extremal multi-center black holes.