Geometry of submanifolds via moving frames
Consider, for simplicity, a 2-dimensional manifold \(\Sigma\) embedded into the 3-dimensional Riemannian manifold \((M,g)\), in such a way that the given frame \(\{e_1,e_2,e_3\}\) is adapted to \(\Sigma\), i.e., \(\omega^3|_{T\Sigma}=0\), where \(\omega^1,\omega^2,\omega^3\) is the dual coframe. The surface \(\Sigma\) inherits a Riemannian metric from the ambient manifold, with its corresponding Levi-Civita connection. We will denote by \(\tilde{\omega}^1,\tilde{\omega}^2\) the restrictions of \(\omega^1,\omega^2\) to \(T\Sigma\), and by \(\tilde{\Theta}\) and \(\tilde{\Omega}\) the connection forms and the curvature forms, respectively, of the inherited connection. According to Cartan's first structural equation , \[ d\omega^j=\omega^i \wedge \Theta^j_{\,\,i}. \] By restricting to \(\Sigma\) (and by the uniqueness of \(\tilde{\Theta}\)) we conclude that \[ \tilde{\Theta}^i_{\,\,j}=\Theta^i_{\,\,j} |_{T\Sigma}, \quad i,j=1,2, \] and that \[ 0= \tilde{\omega}^1\wedge\Theta^3_{\,\,1}