Finite-presentation conjecture for the double dimer group

Let GG be the group generated by the matrices associated with the three tile-edge types XX, YY, and ZZ, and let II be the 2×22\times2 identity matrix. Let I-I be the 2×22\times2 matrix with 1-1 on the diagonal. Using the boundary-word notation for products of these generators, the conjecture concerns finite presentations of GG.

Finite-presentation conjecture. GG has either of the following two equivalent finite presentations:

G=X,Y,Z|[XXX]=[yXyX]=[yzYX]=[YzzX]=[yZxYzX]=IG = \left\langle X, Y, Z \mathrel{\middle|} \downharpoonleft\hspace{-1.2ex}[{XXX}]\hspace{-1.2ex}\upharpoonright = \downharpoonleft\hspace{-1.2ex}[{yXyX}]\hspace{-1.2ex}\upharpoonright = \downharpoonleft\hspace{-1.2ex}[{yzYX}]\hspace{-1.2ex}\upharpoonright = \downharpoonleft\hspace{-1.2ex}[{YzzX}]\hspace{-1.2ex}\upharpoonright = \downharpoonleft\hspace{-1.2ex}[{yZxYzX}]\hspace{-1.2ex}\upharpoonright = -I \right\rangle

or, equivalently,

G=X,Y,Z|[ZxxYXXX]=[yZyZYYzYX]=[ZZxZYzXzX]=I,[XXX]=[yZxYzX]=[yXyX]=I.G = \left\langle X, Y, Z \mathrel{\middle|} \begin{matrix} \downharpoonleft\hspace{-1.2ex}[{ZxxYXXX}]\hspace{-1.2ex}\upharpoonright = \downharpoonleft\hspace{-1.2ex}[{yZyZYYzYX}]\hspace{-1.2ex}\upharpoonright = \downharpoonleft\hspace{-1.2ex}[{ZZxZYzXzX}]\hspace{-1.2ex}\upharpoonright = I, \\ \downharpoonleft\hspace{-1.2ex}[{XXX}]\hspace{-1.2ex}\upharpoonright = \downharpoonleft\hspace{-1.2ex}[{yZxYzX}]\hspace{-1.2ex}\upharpoonright = \downharpoonleft\hspace{-1.2ex}[{yXyX}]\hspace{-1.2ex}\upharpoonright = -I \end{matrix} \right\rangle.

The presentation is motivated by reducing the remaining open boundary words using the bone and snake relations. The source presents this as the group description in the conjecture and does not state a proof or resolution.

Sources & referencesView supporting material

Primary source

Leigh Foster, “Stones, Bones, and Snakes: Tilability of the hexagonal grid via the double dimer model”, arXiv:2509.21700 (2025).

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