tunnel number

《Mathematical Physics: Classical Mechanics》 · 第 164 页 · Chapter 14，应用段中 tunnel number of a knot 的括号定义

The tunnel number is the minimal number of arcs that can be embedded in the knot exterior \(X\), with each arc meeting \(\partial X\) at its end points, so that \(X\) less a regular neighbourhood of the arcs is a handlebody.