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:

• 

  Trefoil knot
  unknotting number 1

• 

  Figure-eight knot
  unknotting number 1

• 

  Cinquefoil knot
  unknotting number 2

• 

  Three-twist knot
  unknotting number 1

• 

  Stevedore knot
  unknotting number 1

• 

  6₂ knot
  unknotting number 1

• 

  6₃ knot
  unknotting number 1

• 

  7₁ knot
  unknotting number 3

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.)

Other numerical knot invariants[]

• Crossing number
• Bridge number
• Linking number
• Stick number

See also[]

• Unknotting problem

References[]

External links[]

• "Three_Dimensional_Invariants#Unknotting_Number", The Knot Atlas.

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