Chromatic Homotopy Theory
《Chromatic Homotopy Theory》 · 第 278 页 · Section 9.2.4 末尾（四维光滑版本）
The situation in higher dimensions is much more problematic: it is still unknown whether every smooth 3-sphere in \(\mathbb R^4\) bounds a smooth 4-disc (this is usually called the Schönflies problem).