# Carlo Sequin

Professor

University of California, Berkeley

Berkeley, CA, USA

My 2018 presentation pays homage to Charles O. Perry. I study his "Topological Ribbon Sculptures" and analyze the mathematical classification of these 2-manifold surfaces. I also analyze the graphs that result, when all the ribbons are skeletonized into simple, thin wires. Perry’s ribbon sculptures represent 1-sided as well as 2-sided surfaces, with a wide variation in their genus and their numbers of borders. But, surprisingly, the skeleton graphs of all the sculptures that I have studied turn out to be planar, i.e., they can be drawn without any self-intersections on a flat sheet of paper.

Using custom CAD software, I have reproduced some of his sculptures and have also created new models, similar in style, but with new topologies.

Using custom CAD software, I have reproduced some of his sculptures and have also created new models, similar in style, but with new topologies.

Curly Utility Trefoil

20 x 20 x 15 cm

3D-print, PLA

2018

This is the result of my attempt to make a Perry-style sculpture based on the simplest non-planar graph: the "Utility Graph," also known as the bipartite graph K_3,3. I found a nice symmetrical way to embed this graph in 3D Euclidean space, by starting with a simple trefoil knot and adding three cross-connecting struts at the points where the typical depiction of this knot would show three crossings. A curled cross section, which is found in many of Perry’s ribbon sculptures, is swept along the trefoil, and the cross-connecting struts form the type of loopy T-intersections with this ribbon, which are also prominent features in Perry's “Continuum” in Washington, DC.

Tetrahedral Trefoil Tangle

14 x 21 x 18 cm

3D-print, PLA

2018

In this sculpture, I follow Perry’s approach that he used in most of his topological sculptures: Nice smooth 3D space curves are first defined (e.g., with the help of steel cables) and these then form the borders of the final 2-manifold. With the borders defined in space, a nice surface is fitted in between; it often assumes the shape of a minimal surface, as one finds in soap films suspended between wires. Here, I start with four identical trefoil loops and place them onto the faces of a tetrahedron, letting them interlink across the edges of the tetrahedron. Then fill in a soap-film like surface defined by the given trefoil borders. The result is a single-sided surface of genus 10 with (of course) four borders.