writhe

《Symplectic Geometry》 · 第 62 页 · Section 3.6，Invariants from oriented diagrams

Let \(c\) be a crossing in an oriented diagram \(D\) and define \(\varepsilon(c)=+1\) if \(c\) is a positive crossing, \(-1\) if \(c\) is a negative crossing. If we write “\(c\in D\)” to mean “\(c\) is a crossing in \(D\)” then the writhe of \(D\) is defined as \(w(D)=\sum_{c\in D}\varepsilon(c)\).