Mathematical Physics: Classical Mechanics
《Mathematical Physics: Classical Mechanics》 · 第 7 页 · Chapter 1，unknotting number 的定义
The unknotting number \(u(K)\) of a knot \(K\) is likewise a popular but intractable invariant; it will be mentioned in Chapter 7. By definition, \(u(K)\) is the minimum number of crossing changes (from “over” to “under” or vice versa) needed to change \(K\) to the unknot, where the minimum is taken over all possible sets of crossing changes in all possible diagrams of \(K\).