The polytopes $\mathcal{ U}_{ I,\bar{ J}}$ were introduced by Ceballos, Padrol, and Sarmiento to provide a geometric approach to the study of $(I,\bar{ J})$-Tamari lattices. They observed a connection between certain $\mathcal{ U}_{ I,\bar{ J}}$ and acyclic root polytopes, and wondered if M\'esz\'aros' subdivision algebra can be used to subdivide all $\mathcal{ U}_{ I,\bar{ J}}$. We answer this in the affirmative from two perspectives, one using flow polytopes and the other using root polytopes. We show that $\mathcal{ U}_{ I,\bar{ J}}$ is integrally equivalent to a flow polytope that can be subdivided using the subdivision algebra. Alternatively, we find a suitable projection of $\mathcal{ U}_{ I,\bar{ J}}$ to an acyclic root polytope which allows subdivisions of the root polytope to be lifted back to $\mathcal{ U}_{ I,\bar{ J}}$. As a consequence, this implies that subdivisions of $\mathcal{ U}_{ I,\bar{ J}}$ can be obtained with the algebraic interpretation of using reduced forms of monomials in the subdivision algebra. In addition, we show that the $(I,\bar{ J})$-Tamari complex can be obtained as a triangulated flow polytope.
Categories: math.CO