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