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The property of an immersed submanifold of a Riemannian space defined by the fact that each of its isometric deformations (cf. Infinitesimal deformation) is trivial, that is, the corresponding velocity field on is induced by a Killing vector field on : , where is the isometric immersion of into . The question of the rigidity of a submanifold — essentially that of the uniqueness of the solution of the system of differential equations that are the linearizations of the fundamental system of equations in the theory of surfaces — has practically never been considered in the case where and ; however, in the simplest situation , it has been possible to construct a more-or-less complete theory for surfaces of positive curvature contained in spaces of constant curvature (see Vekua method). Only isolated results are known about the rigidity of surfaces of non-positive or mixed curvature; it turns out that the degree of regularity of the deformations under discussion has influence on the rigidity of a surface, apart from its shape in space.
As a rule, a non-closed surface is non-rigid, but: a) examples have been made of surfaces with a flat point every neighbourhood of which is rigid or allows an infinitesimal deformation of bounded regularity; and b) there exist rigid non-closed convex surfaces of total curvature , bordered by plane parabolic curves (parts of surfaces of type ).
The extent of the restriction of the mobility of the boundary of the surfaces or of curves inside it influences the rigidity of a surface; for example: 1) spherical segments sliding along a plane will be rigid or not according to as a hemisphere is smaller or larger than ; 2) a piece of a hyperbolic paraboloid with two intersecting fixed generators is rigid; and 3) a piece of the plane with a fixed boundary is not rigid.
Closed surfaces have been studied in more detail from the point of view of their rigidity; for example, ) a closed convex surface is rigid (see the Blaschke–Weyl formula, and also ); ) at the same time, there are non-rigid closed surfaces of revolution of mixed curvature; ) a torus is rigid; ) a closed cylindroid is rigid if and only if the area of the mean section satisfies the equation
where and are the areas of the upper and lower bases; and ) the metric product of two-dimensional spheres is rigid in the Euclidean space and not rigid in for .
The concept of rigidity here defined is sometimes called first-order rigidity. Rigidities of second and higher orders have also been introduced. The concept of rigidity carries over to non-regular surfaces, for example, to polyhedra; however, the main results there relate to convex polyhedra (see Cauchy theorem on polyhedra), and to surfaces in Riemannian spaces, for example, closed surfaces of arbitrary genus and of positive extremal curvature are rigid.
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Rigidity. Encyclopedia of Mathematics. URL: http://www.encyclopediaofmath.org/index.php?title=Rigidity&oldid=26123