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Lattice points of flow polytopes related to caracol graphs
Electronic Research Archive 2025, 33(10): 6141-6175
Published: 20 October 2025
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Flow polytopes are fundamental objects in algebraic combinatorics. In this paper, we study the enumeration of lattice points in flow polytopes associated with ( a 1 , a 2 )-caracol graphs on n + 2 vertices. Our main result establishes a closed-form expression for the number of lattice points by constructing an explicit combinatorial bijection between Dyck paths and the integer lattice points of the two-parameter family of polytopes ( a 1 , a 2 )-Car n + 1 , using pseudo-ladder diagrams together with vector partition techniques. When a 2 = 1, the lattice point sequence of caracol polytopes coincides with the OEIS sequence A126216 (The On-Line Encyclopedia of Integer Sequences), which enumerates Schröder paths of semilength n with exactly k peaks. Furthermore, we establish a bijection between Schröder paths and the integer lattice points of the two-parameter family of polytopes ( a 1 , a 2 )-Car n + 1 . All bijections are implemented as explicit algorithms in Python, with the complete source code provided in the appendix to ensure reproducibility.

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