
VICTOR ALEXANDROV, Sobolev Institute of Mathematics, Novosibirsk, 630090 Russia 
Sufficient conditions for the extendability of an Nth order flex of polyhedra 
A very longstanding (and still open) problem is to prove that a smooth compact surface in Euclidean 3space is rigid. Classical attempts to solve this problem were based on the notion of an infinitesimal flex. Since a counterexample was constructed by R. Connelly in the class of polyhedral surfaces, particular attention was given to studying infinitesimal flexes of polyhedra.
We give a new approach to describing a high order flex of polyhedra. The main results provide some sufficient conditions under which an infinitesimal flex of a polyhedron can be extended to an authentic flex. We discuss also results of computer experiments demonstrating that one set of our sufficient conditions is fulfilled for the Bricard octahedra.
More precisely, let Q be a closed polyhedron with v vertexes, eedges and triangular faces. A special bilinear map
is constructed in such a way that
the vector
Theorem. Let a polyhedron Q=X_{0} has an infinitesimal flex
and let
be the linear span of the
vectors
. Suppose the equation
A_{0}X=B(X_{i},
X_{j})B(X_{j},X_{i}) has a solution for all
,
. Then Q is flexible.
Computer experiments show that the conditions of the Theorem are fulfilled with n=5 for Bricard's octahedron of the second type.