A simple analog involves a 2D surface made out of triangles whose edges have integer lengths j i . … In the early 1990s there was again interest in spin networks when the Turaev–Viro invariant for 3D spaces was discovered from a topological field theory involving triangulations weighted with 6j symbols of the quantum group SU(2) q —and it was seen that invariance under Alexander moves on the triangulation corresponded to the Biedenharn–Elliott identity for 6j symbols.