Unknotting number

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]

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

Whitehead link being unknotted by undoing one crossing

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.)