Family of mathematical knots
In knot theory, a branch of mathematics, a twist knot is a knot obtained by repeatedly twisting a closed loop and then linking the ends together. (That is, a twist knot is any Whitehead double of an unknot.) The twist knots are an infinite family of knots, and are considered the simplest type of knots after the torus knots.
Construction.
A twist knot is obtained by linking together the two ends of a twisted loop. Any number of half-twists may be introduced into the loop before linking, resulting in an infinite family of possibilities. The following figures show the first few twist knots:
Properties.
All twist knots have unknotting number one, since the knot can be untied by unlinking the two ends. Every twist knot is also a 2-bridge knot. Of the twist knots, only the unknot and the stevedore knot are slice knots. A twist knot with formula_1 half-twists has crossing number formula_2. All twist knots are invertible, but the only amphichiral twist knots are the unknot and the figure-eight knot.
Invariants.
The invariants of a twist knot depend on the number formula_1 of half-twists. The Alexander polynomial of a twist knot is given by the formula
formula_4
and the Conway polynomial is
formula_5
When formula_1 is odd, the Jones polynomial is
formula_7
and when formula_1 is even, it is
formula_9
References.
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