The term twenty-vertex entropy constant is used here for the asymptotic growth constant of Eulerian orientations of the triangular
lattice. Let be the lattice graph on an
period of the triangular lattice with opposite
sides identified, where
.
Let
count its graph orientations in which every
graph vertex has three inwardly directed and three
outwardly directed edges. These are Eulerian
orientations, with
possible local configurations at each graph vertex.
Baxter (1969) showed that
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(1)
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(2)
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(3)
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(OEIS A104956; Finch 2003, p. 412).