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# Unknotting number

Trefoil knot without 3-fold symmetry being unknotted by one crossing switch.
Whitehead link being unknotted by undoing one crossing

In the mathematical area of knot theory, the unknotting number of a knot is the minimum number of times the knot must be passed through itself (crossing switch) to untie it. If a knot has unknotting number ${\displaystyle n}$, then there exists a diagram of the knot which can be changed to unknot by switching ${\displaystyle n}$ crossings.[1] The unknotting number of a knot is always less than half of its crossing number.[2]

Any composite knot has unknotting number at least two, and therefore every knot with unknotting number one is a prime knot. The following table show the unknotting numbers for the first few knots:

In general, it is relatively difficult to determine the unknotting number of a given knot. Known cases include:

• The unknotting number of a nontrivial twist knot is always equal to one.
• The unknotting number of a ${\displaystyle (p,q)}$-torus knot is equal to ${\displaystyle (p-1)(q-1)/2}$.[3]
• The unknotting numbers of prime knots with nine or fewer crossings have all been determined.[4] (The unknotting number of the 1011 prime knot is unknown.)

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• Knot Floer homology - Prof. Peter Ozsváth

## Other numerical knot invariants

3. ^ "Torus Knot", Mathworld.Wolfram.com. "${\displaystyle {\frac {1}{2}}(p-1)(q-1)}$".