Mathematical Physics: Classical Mechanics
《Mathematical Physics: Classical Mechanics》 · 第 113 页 · Chapter 11，Theorem 11.5（完整陈述）
Suppose that \(M\) is an orientable 3-manifold and that there exists a map \(S^2\longrightarrow M\) that is not homotopic to a constant map (that is, \(\Pi_2(M)\neq0\)). Then there exists a (piecewise linear) embedding \(S^2\longrightarrow M\) that is not homotopic to a constant map.