• Xiaowei Xu

      Articles written in Proceedings – Mathematical Sciences

    • Holomorphic Two-Spheres in Complex Grassmann Manifold $G(2, 4)$

      Xiaowei Xu Xiaoxiang Jiao

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      In this paper, we use the harmonic sequence to study the linearly full holomorphic two-spheres in complex Grassmann manifold $G(2,4)$. We show that if the Gaussian curvature 𝐾 (with respect to the induced metric) of a non-degenerate holomorphic two-sphere satisfies $K\leq 2$ (or $K\geq 2$), then 𝐾 must be equal to 2. Simultaneously, we show that one class of the holomorphic two-spheres with constant curvature 2 is totally geodesic. Concerning the degenerate holomorphic two-spheres, if its Gaussian curvature $K\leq 1$ (or $K\geq 1$), then $K=1$. Moreover, we prove that all holomorphic two-spheres with constant curvature 1 in $G(2,4)$ must be $U(4)$-equivalent.

    • On Conformal Minimal 2-Spheres in Complex Grassmann Manifold $G(2,n)$

      Jie Fei Xiaoxiang Jiao Xiaowei Xu

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      For a harmonic map 𝑓 from a Riemann surface into a complex Grassmann manifold, Chern and Wolfson [4] constructed new harmonic maps $\partial f$ and $\overline{\partial} f$ through the fundamental collineations 𝜕 and $\overline{\partial}$ respectively. In this paper, we study the linearly full conformal minimal immersions from $S^2$ into complex Grassmannians $G(2,n)$, according to the relationships between the images of $\partial f$ and $\overline{\partial}f$. We obtain various pinching theorems and existence theorems about the Gaussian curvature, Kähler angle associated to the given minimal immersions, and characterize some immersions under special conditions. Some examples are given to show that the hypotheses in our theorems are reasonable.

    • Holomorphic Two-Spheres in the Complex Grassmann Manifold $G(k, n)$

      Xiaoxiang Jiao Xu Zhong Xiaowei Xu

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      In this paper, we study the non-degenerate holomorphic $S^2$ in the complex Grassmann manifold $G(k,n), 2k\leq n$, by the method of moving frame. For a non-degenerate holomorphic one, there exists globally defined positive functions $\lambda_1,\ldots,\lambda_k$ on $S^2$. We first show that the holomorphic $S^2$ in $G(k, 2k)$ is totally geodesic if these $\,\lambda_i$ are all equal. Conversely, for any totally geodesic immersion 𝑓 from $S^2$ into $G(k, n)$, we prove that $f(S^2)\subset G(k, 2k)$ up to $U(n)$-transformation, $\lambda_i=\frac{1}{\sqrt{k}}$, the Gaussian curvature $K=\frac{4}{k}$ and 𝑓 is given by $(z_0,z_1)\mapsto(z_0 I_k,z_1 I_k,0)$, in terms of homogeneous coordinate.

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