Lyapunov surfaces and curves

A surface in the three-dimensional Euclidean space is called a Lyapunov surface if it satisfies the following three conditions (Lyapunov's conditions): 1) at every point of there is a well-defined tangent plane, and consequently a well-defined normal; 2) there is a number , the same for all points of , such that if one takes the part of lying inside the Lyapunov sphere with centre at an arbitrary point and radius , then the lines parallel to the normal to at meet at most once; and 3) there are two numbers and , , the same for the whole of , such that for any two points , (*)
where is the angle between the normals to at and . Sometimes these three conditions are supplemented by the requirement that is closed and that the solid angle under which any part of is visible at an arbitrary point is uniformly bounded.
The Lyapunov conditions can be generalized to hypersurfaces in , .
Similarly, a simple continuous curve in the plane is called a Lyapunov curve if it satisfies the following conditions: ) at every point of there is a well-defined tangent, and consequently a well-defined normal; and ) there are two numbers and , , the same for the whole of , such that for any two points (*) holds, where is the angle between the tangents or normals to at and . Here Lyapunov's condition 2) follows from ) and ). The Lyapunov curves are a subclass of the simple smooth curves.