Add the tridiminished icosahedron rigidity problem

Test-problems.md

@@ -140,6 +140,19 @@ There are various ways to represent the solid and to constrain the faces to be r
 - Tridiminished icosahedron (J63)
 - Snub disphenoid (J84)
 
+### Tridiminished icosahedron rigidity
+
+#### Statement
+
+Take the 1-skeleton (vertices and edges) of a tridiminished icosahedron, keeping the edges fixed at unit length but allowing the angles between them to vary. This assembly is flexible. Now, constrain each pentagonal face to be planar. Is this enough to make the assembly rigid?
+
+#### Notes
+
+The answer *yes* is suggested by:
+
+- Glen's physical experiments with Geometiles, using chains of squares to represent the planar pentagonal faces.
+- Aaron's numerical experiments with dyna3, using the version of the engine from pull request #92.
+
 ### 5-5-4 acrohedron
 
 #### Source