Forceless Sadowsky strips are spherical

Existence of Optimal Flat Ribbons

We apply the direct method of the calculus of variations to show that any nonplanar Frenet curve in R 3 can be extended to an infinitely narrow flat ribbon having minimal bending energy. We also show that, in general, minimizers are not free of planar points, yet such points must be isolated under the mild condition that the torsion does not vanish.