设正向路径\gamma_+ \subset \mathcal{M}_{\text{MB}}沿莫比乌斯带中轴的正向环绕，反向路径\gamma_- \subset \mathcal{M}_{\text{MB}}沿反向环绕。2. 同伦型： \mathcal{M}_{\text{MB}} \simeq S^1 ，基本群\pi_1(\mathcal{M}_{\text{MB}}) \cong \mathbb

虚边：$E_{\text{virtual}} = \{(a,v_0),(b,v_0),(c,v_0),(d,v_0),(e,v_0),(f,v_0)\}$环边：$E_{\text{ring}} = \{(a,b),(b,c),(c,d),(d,e),(e,f),(f,a)\}$G = (V, E) \quad \text{其中} \quad V = \{a,b,c,d,e,f,v_0\}对角线：$