Topology in Molecular Biology

80 L.H. Kauffman and S. Lambropoulou

Fig. 5.11.Reducing to the alternating form

Recall that a tangle is alternating if and only if it has crossings all of the
same type. Thus,a rational tangleT =[[a 1 ],[a 2 ],...,[an]]is alternating if
theai’s are all positive or all negative. For example, the tangle of Fig. 5.17 is
alternating.
A rational tangleT=[[a 1 ],[a 2 ],...,[an]] is said to be incanonical formif
Tis alternating andnis odd. The tangle of Fig. 5.17 is in canonical form. We
note that ifTis alternating andneven, then we can bringTto canonical form
by breakinganby a unit, e.g. [[a 1 ],[a 2 ],...,[an]] = [[a 1 ],[a 2 ],...,[an−1],[1]],
ifan>0.
The last key observation is the following well-known fact about continued
fractions.

Lemma 3.Every continued fraction[a 1 ,a 2 ,...,an]can be transformed to a
unique canonical form[β 1 ,β 2 ,...,βm],where allβi’s are positive or all nega-
tive integers andmis odd.

Proof.It follows immediately from Euclid’s algorithm. We evaluate first
[a 1 ,a 2 ,...,an]=p/q,and using Euclid’s algorithm we rewritep/qin the de-
sired form. We illustrate the proof with an example. Suppose thatp/q=11/7.
Then
11
7

=1+

4

7

=1+

1

7

4

=1+

1

1+^34

=1+

1

1+^14

3

=1+

1

1+1+^11

3

=[1, 1 , 1 ,3] = 1 +

1

1+1+^11

2+^11

=[1, 1 , 1 , 2 ,1].

This completes the proof.

Note that ifT =[[a 1 ],[a 2 ],...,[an]] andS=[[b 1 ],[b 2 ],...,[bm]] are ra-
tional tangles in canonical form with the same fraction, then it follows from
this lemma that [a 1 ,a 2 ,...,an] and [b 1 ,b 2 ,...,bm] are canonical continued
fraction forms for the same rational number, and hence are equal term by
term. Thus the uniqueness of canonical forms for continued fractions im-
plies the uniqueness of canonical forms for rational tangles. For example, let
T= [[2],[−3],[5]].ThenF(T)=[2,− 3 ,5] = 23/ 14 .But 23/14 = [1, 1 , 1 , 1 ,4],
thusT∼[[1],[1],[1],[1],[4]], and this last tangle is the canonical form ofT.