Past exam of the mathematics course of the University of Cambridge 2007 iii Paper 24 3 c Solution Created 2026-10-03 Updated 2026-10-14
Use the standard closed three-strand braid , a four-crossing diagram of the figure-eight knot. The diagram has two positive and two negative crossings, so .
Four-crossing closed braid for the figure-eight knot, with three starting meridians
. Details. In the Temperley-Lieb diagram algebra on three strands put , . The relations are , , , . The bracket image of one two-crossing block isClosing planar diagrams gives the bracket trace on the three-strand Temperley-Lieb algebra:These values follow because the closures have respectively three, two and one circles. Expanding and reducing givesTaking its trace and substituting yieldsThere is no writhe correction, soThe equality is immediate from the five displayed coefficients. The same braid model supplies the meridians used in part (d).
