Analysis of the first variation and a numerical gradient flow for integral Menger curvature

  • Analysis der ersten Variation und ein numerischer Gradientenfluss für „integral Menger curvature”

Hermes, Tobias; von der Mosel, Heiko (Thesis advisor)

Aachen : Publikationsserver der RWTH Aachen University (2012)
Dissertation / PhD Thesis

Aachen, Techn. Hochsch., Diss., 2012


In this thesis, we consider the knot energy "integral Menger curvature" which is the triple integral over the inverse of the classic circumradius of three distinct points on the given knot to the power $pin [2,infty)$. We prove the existence of the first variation for a subset of a certain fractional Sobolev space if p>3 and for a subset of a certain Hölder space otherwise. We also discuss how fractional Sobolev and Hölder spaces can be generalised for 2pi-periodic, closed curves. Since this energy is not invariant under scaling, we additionally consider a rescaled version of the energy, where we take the energy to the power one over p and multiply by the length of the curve to a certain power. We prove that a circle is at least a stationary point of the rescaled energy. Furthermore, we show that in general a functional with a scaling behaviour like $E(rgamma)=r^alpha E(gamma)$, $alphainR$ cannot have stationary points unless alpha=0. Consequently "integral Menger curvature" for $peq 3$ can be used as a subsidiary condition for a Lagrange-multiplier rule. We consider a numerical gradient flow for the rescaled energy. For this purpose we use trigonometric polynomials to approximate the knots and the trapezoidal rule for numerical integration, which is very efficient in this case. Moreover, we derive a suitable representation of the first variation. We present an algorithm for the adaptive choice of time step size and for the redistribution of the Fourier coefficients. After discussing the full discretization we present a wide collection of example flows.