Brunnian 4-strand braid
Another topological picture today, this time courtesy of Ryan Budney in Victoria, Canada. He says that he was in Rochester working with Fred Cohen and needed to write down an explicit example of a Brunnian link. That is, a linked set of knots such that if any of the knots are removed then the rest of the knots are completely unlinked with each other. (This is the picture on the left of the board.) The other pictures on the board are this Brunnian link drawn as a braid. Apparently this particular braid is a double commutator.