Nonrigidity of flat ribbons

Abstract

We study ribbons of vanishing Gaussian curvature, i.e. flat ribbons, constructed along a curve in ℝ³. In particular, we first investigate to which extent the ruled structure determines a flat ribbon: in other words, we ask whether for a given curve γ and ruling angle (angle between the ruling line and the curve's tangent) there exists a well-defined flat ribbon. It turns out that the answer is positive only up to an initial condition, expressed by a choice of normal vector at a point. We then study the set of infinitely narrow flat ribbons along a fixed curve γ in terms of energy. By extending a well-known formula for the bending energy of the rectifying developable, introduced in the literature by Sadowsky in 1930, we obtain an upper bound for the difference between the bending energies of two solutions of the initial value problem. We finally draw further conclusions under some additional assumptions on the ruling angle and the curve γ.