A robot that unknots knots

Abstract: Consider a robot that walks along a knot once on a knot diagram and switches every undercrossing it meets, stopping when it comes back to the starting position. We show that such a robot always unknots the knot. In fact, we prove that the robot produces an ascending diagram, and we provide a purely combinatorial proof that every ascending or descending knot diagram with C crossings can be transformed into the zero-crossing unknot diagram using at most (7C+1)C Reidemeister moves. Moreover, we show that an ascending or descending knot diagram can always be transformed into a zero-crossing unknot diagram using Reidemeister moves that do not increase the number of crossings.