# Carlo Sequin

I like to make mathematical demonstration models that are intriguing and pleasing enough, so one can enjoy them, even if one does not know the mathematical stories underlying them.

For the last few years I have focused on making demonstration models for topological concepts, such as the classification of all 2-manifolds, the symmetry of knots, or the different ways that two identical wire-frames can be interlinked.

With Walt van Ballegooijen we have studied the number of topologically different ways in which two identical tetrahedral wire-frames could be interlinked. Not counting mirror images separately, we have found eleven different configurations. For each topological configuration there is a maximal clearance between the infinitely thin mathematical wire frames. If the edges are expanded to a diameter identical to this clearance, then a rigid structure results. This particular configuration has been scaled up to accommodate fattened edges of 22mm diameter, the size of the PVC pipes employed.