Symplectic Geometry
《Symplectic Geometry》 · 第 98 页 · Section 4.10，Bridge presentations 开头
We can position the link so that it lies entirely in a plane except for a few bridges. The bridges must be simple so we insist that their projection onto the plane consists of disjoint straight lines (see Figure 4.13). Such an embedding of a link is called a bridge presentation. The minimum number of bridges required is a link invariant called the bridge number (or bridge index) of the link and is denoted by \(b(L)\).