Reidemeister's Theorem Essay

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Figure 2.1: TYPE I: Allows the addition and subtraction of a twist addition or subtraction of two crossings Figure 2.2: TYPE II: Allows the removal of two crossings Figure 2.3: TYPE III: Allows a strand of the knot to be slid from one side of the crossing to the other. Definition 2.3 Two projections are called equivalent if one can be changed into the other by a finite number of Reidemeister moves. Reidemeister moves do not change the knot but can be used to manipulate a knot projection to determine if it is equivalent to the trivial knot – or any other defined knot. Theorem 2.1 Two knots are equivalent if and only if all their projections are equivalent Reidemeister’s theorem ensure that moves I, II and III correspond to ambient …show more content…

For each crossing cr in which Di and Dj cross, set εr = +1 if the link is right-handed and εr = −1 if the link is left-handed. Then, the linking number of the two components is lk(D_i,D_j )=1/2 ∑_r▒ε_r . Linking number is a link invariant. The linking number of a splittable two-component link is 0. 2.4 Tri-colourability Another method of distinguishing the unknot from other knots is by using a diagram’s tricolorable property. A projection of a knot or link is tricolorable if each strand can be coloured one of three different colours, at least two colours must be used and at each crossing, the three incident strands are either all different colours or all the same colour. Figures 2. : trefoil knot and 6_1 are tricolorable If the three colours are replaced with integers 0, 1, and 2 and at each crossing, the overcrossing is x and the two other strands are y and z then the concept of the three incident strands being either all different colours or all the same colour can be restated as 2x-y-z=0 (mod 3). If a projection of a knot or link is tricolorable, the Reidemeister Moves will preserve the

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