Chromatic Homotopy Theory
《Chromatic Homotopy Theory》 · 第 275 页 · Section 9.2.2，Definition 9.2.2（三维流形义）
The connected sum \(M=M_1\#M_2\) of two oriented connected 3-manifolds \(M_1,M_2\) is constructed by removing the interiors of two closed balls from \(M_1\) and \(M_2\), and then gluing the two resulting spheres via any orientation-reversing diffeomorphism.