The geometry of fractal sets
《The geometry of fractal sets》 · 第 39 页 · Chapter 4，Theorem 4.1；本书从 Chapter 2 起约定所有对象和映射均在 PL 范畴
DEHN’S LEMMA. Suppose \(M\) is a 3-manifold and \(f:B^2\longrightarrow M\) is a map such that for some neighborhood \(A\) of \(\partial B^2\) in \(B^2\), \(f|A\) is an embedding and \(f^{-1}(f(A))=A\). Then \(f|\partial B^2\) extends to an embedding \(g:B^2\longrightarrow M\).