Rigidity Conditions for the Boundaries of Submanifolds in a Riemannian Manifold
Developing A.D. Aleksandrov's ideas, the first-named author of this article proposed the following approach to study of rigidity problems for the boundary of a $C^0$-submanifold in a smooth Riemannian manifold: Let $Y_1$ be a 2-dimensional compact connected $C^0$-submanifold with nonempty boundary in a 2-dimensional smooth connected Riemannian manifold $(X,g)$ without boundary satisfying the condition $$\rho_{Y_1}(x,y) = \liminf_{x' \to x, y' \to y, x',y' \in \mathop{\rm Int} Y_1} \{[l(\gamma_{x',y',\mathop{\rm Int} Y_1})]\} < \infty,$$ if $x,y \in Y_1$. Here $\inf[l(\gamma_{x',y',\mathop{\rm Int} Y_1})]$ is the infimum of the length of smooth paths joining $x'$ and $y'$ in the interior $\mathop{\rm Int} Y_1$ of $Y_1$. In the present paper, we first establish that $\rho_{Y_1}$ is a metric on $Y_1$. Suppose further that $Y_1$ is strictly convex in the metric $\rho_{Y_1}$. Consider another 2-dimensional compact connected $C^0$-submanifold $Y_2$ of $X$ with boundary satisfying the condition $\rho_{Y_2}(x,y) < \infty$, $x,y \in Y_2$, and assume that $\partial Y_1$ and $\partial Y_2$ are isometric in the metrics $\rho_{Y_j}$, $j = 1,2$. There appears the following natural question: Under which additional conditions are the boundaries $\partial Y_1$ and $\partial Y_2$ of $Y_1$ and $Y_2$ isometric in the metric $\rho_X$ of the ambient manifold $X$? The paper is devoted to the detailed discussions of this question. In it, we in particular obtain a number of new results concerning the rigidity problems for the boundaries of $C^0$-submanifolds in a Riemannian manifold. The case of $\dim Y_j = \dim X = n$, $n > 2$, is also considered.