Topology in Molecular Biology

6 Writhe Versus the Number of Crossings 121

a

b

c

nullification

a (odd)

+1 crossings

b (odd)

+1 crossings

c (even)

-1 crossings

b

a

Fig. 6.4.(a) A rational tangle with three rows, containinga,b,andccrossings
respectively. (b) Nullification of the knot obtained by the closure of the tangle in
the caseaodd,bodd, andceven