Combinatorial Theory

We prove an explicit expression for the solutions of the discrete Schwarzian octahedron recurrence, also known as the discrete Schwarzian KP equation (dSKP), as the ratio of two partition functions. Each one counts weighted oriented dimer configurations of an associated bipartite graph, and is equal to the determinant of a Kasteleyn matrix. This is in the spirit of Speyer's result on the dKP equation, or octahedron recurrence (Journal of Alg. Comb. 2007). One consequence is that dSKP has zero algebraic entropy, meaning that the growth of the degrees of the polynomials involved is only polynomial. There are cancellations in the partition function, and we prove an alternative, cancellation free explicit expression involving complementary trees and forests. Using all of the above, we show several instances of the Devron property for dSKP, i.e., that certain singularities in initial data repeat after a finite number of steps. This has many applications for discrete geometric systems and is the subject of a companion paper (preprint 2022, Affolter, de Tillière, and Melotti). We also find limit shape results analogous to the arctic circle of the Aztec diamond. Finally, we discuss the combinatorics of all the other octahedral equations in the classification of Adler, Bobenko and Suris (IMRN 2012).

Mathematics Subject Classifications: 05A15, 37K10, 37K60, 82B20, 82B23

Keywords: Dimer model, octahedron recurrence, discrete KP equation, integrable system, spanning forests, algebraic entropy, discrete geometry, projective geometry, Aztec diamond, limit shapes