Projective invariance of shaky structures

SummaryA framework consisting of rigid rods which are connected in freely moveable knots, in general is stable if the number of knots is sufficiently large. In exceptional cases, however, the rodwork may allow an infinitesimal deformation. Due to a theorem of Liebmann, this apparently metric property of existing shakiness in fact is a projective one, as it does not vanish if the structure is transformed by an affine or projective collineation. The paper presents a new analytic proof of this remarkable phenomenon. The developments are applicable also to polyhedra with rigid plates and to closed chains of rigid links.