Existence and Regularity for a Curvature Dependent Variational Problem

It is proved that smooth closed curves of given length minimizing the principal eigenvalue of the Schr"odinger operator $-\frac{d^2}{d{s}^2}+{\kappa }^2$ exist. Here $s$ denotes the arclength and $\kappa$ the curvature. These minimizers are automatically planar, analytic, convex curves. The straight segment, traversed back and forth, is the only possible exception that becomes admissible in a more generalized setting. In proving this, we overcome the difficulty from a lack of coercivity and compactness by a combination of methods.