On the generalised ideal flow of closed planar curves

Abstract

For each integer $m\ge0$ we study the $m$-ideal energy \[ E_m[\gamma]:=\frac12\int_\gamma k_{s^m}^2\,ds \] on closed immersed planar curves, where $k$ is signed curvature and $s$ is arclength; $k^2_{s^m} := (k_{s^m})^2$. The $m$-ideal energies contain Euler's elastic energy and the Dirichlet energy for the curvature scalar as special cases ($m=0,1$). We completely classify the closed smooth critical points of $E_m$ for all $m\ge1$: they are precisely the round multiply-covered circles. For the steepest descent $L^2(ds)$-gradient flow of $E_m$, the \emph{$m$-ideal flow}, we prove that for each nonzero turning number there is a curvature-oscillation threshold such that every canonical relaxed flow starting from $W^{2,2}$ initial data below this threshold is immortal and exponentially asymptotic in the smooth topology to a round multiply-covered circle. We also prove that every immortal canonical relaxed trajectory with bounded unnormalised length converges to the corresponding circle. We furthermore treat rough initial data of class $W^{2,2}$; such data typically has infinite $E_m$ energy when $m\ge1$. In the small-curvature-oscillation basin, every such curve generates a unique canonical relaxed length-normalised flow, smooth for every positive time, continuously dependent on the initial data, and smoothly convergent to the multiply-covered circle. These results are known in the $m=0$ case, substantially strengthen existing work in the $m=1$ case, and are new for $m>1$.