Higher Algebra
《Higher Algebra》 · 第 82 页 · Chapter 2，Gluck twists 说明中的 knotted 定义
Let \(S\) be a 2-sphere embedded in \(S^4\). […] A sphere \(S\) in \(S^4\) is called knotted if it does not bound an embedded 3-ball in \(S^4\).