Mathematical Physics: Classical Mechanics
《Mathematical Physics: Classical Mechanics》 · 第 86 页 · Chapter 8，Definition 8.11 及 flat 的邻域条件（拓扑切片义）
A knot \(K\subset S^3\) is a slice knot if there is a flat disc \(D\) contained in \(B^4\) such that \(K=\partial D=D\cap S^3\). Such a disc is called a slicing disc for \(K\). Here “flat” means that \(D\) has a neighbourhood \(N\) that is a copy of \(D\times I^2\) meeting \(S^3\) in \(\partial D\times I^2\) (of course, \(I^2=I\times I\), and this is just another disc).