April 27, 2016
The "trefoil" knot.
image credit: KnotInfo web site
Not a Knot
Another Simple Knot
A better way was discovered by Reidemeister in the 1930's.
Any isotopy between knots can be realized as a finite sequence of three types of small changes.
Those changes are now called Reidemeister Moves.
Reidemeister Moves of type I
Reidemeister Moves of type II
Reidemeister Moves of type III
Arguments are often pleasantly pictorial. These two knot projections represent the same knot.
Part one of the argument
Part two of the argument
image credit: KnotInfo web site
image credit: Wikipedia
An "overpass" is an arc of the knot which only has the overstrand in all of its crossings. A "maximal overpass" is an overpass that is not part of any larger overpass.
The bridge number of a knot is the minimum number of maximal overpasses, taken over all possible planar projection drawings of the knot.
Introduced and studied by Horst Schubert, a student of Seifert and an important figure in mid-1900's knot theory.
Bridge Number equal to \(2\).
Bridge Number equal to \(2\).
Bridge Number equal to \(3\).
If a knot \(K\) has bridge number equal to \(1\), then \(K\) is the "unknot".
A knot \(K\) has bridge number equal to \(2\) exactly when it can be realized as the closure of a "rational tangle."
Both due to Schubert, restated after Conway's work
This is the numerator closure of \(65/38\).
If \(K\) is a Montesinos knot, then the bridge number of \(K\) is equal to the number of rational tangle pieces in the representation.
Of the 552 prime knots with 11 crossings, 91 are rational knots and hence have bridge number equal to 2, and 15 are Montesinos knots with 4 tangles and hence have bridge number equal to 4. The rest have bridge number equal to 3.
Can we understand and classify the knots with bridge number 3?
Can we find an algorithm for computing a minimal bridge representation of a knot from any planar projection diagram?
Can we compute the bridge numbers of (some of) the 2176 prime knots with 12 crossings?
Let K be a knot, with a given planar projection drawing K.
The part that you repeat:
Running through the crossings that are not part of the current bridge set, in increasing order of label:
If the bit above in the loop cannot be done at all, take the crossing with the smallest remaining number and declare its overpassing arc as another bridge.
\(K\) | \(b(K)\) | || | \(K\) | \(b(K)\) | || | \(K\) | \(b(K)\) |
\(12a_{0001}\) | 3, 4 | || | \(12a_{0008}\) | 3 | || | \(12a_{0015}\) | 3, 4 |
\(12a_{0002}\) | 3, 4 | || | \(12a_{0009}\) | 3 | || | \(12a_{0016}\) | 3 |
\(12a_{0003}\) | 3 | || | \(12a_{0010}\) | 3 | || | \(12a_{0017}\) | 3 |
\(12a_{0004}\) | 3 | || | \(12a_{0011}\) | 3, 4 | || | \(12a_{0018}\) | 3 |
\(12a_{0005}\) | 3 | || | \(12a_{0012}\) | 3 | || | \(12a_{0019}\) | 3 |
\(12a_{0006}\) | 3, 4 | || | \(12a_{0013}\) | 3, 4 | || | \(12a_{0020}\) | 3 |
\(12a_{0007}\) | 3, 4 | || | \(12a_{0014}\) | 3 | || | \(12a_{0021}\) | 3, 4 |