Folding the square-diagonal lattice

We study the problem of "phantom" folding of the two-dimensional square lattice, in which the edges and diagonals of each face can be folded. The nonvanishing thermodynamic folding entropy per face s ≃ .2299(1) is estimated both analytically and numerically, by successively mapping the model onto a dense loop model, a spin model and a new 28 Vertex, 4-color model. Higher-dimensional generalizations are investigated, as well as other foldable lattices.