Parametrization invariance and shape equations of elastic axisymmetric vesicles.

The issue of different parameterizations of the axisymmetric vesicle shape addressed by Hu Jian-Guo and Ou-Yang Zhong-Can [ Phys.Rev. E 47 (1993) 461 ] is reassesed, especially as it transpires through the corresponding Euler Lagrange equations of the associated elastic energy functional. It is argued that for regular, smooth contours of vesicles with spherical topology, different parameterizations of the surface are equivalent and that the corresponding Euler Lagrange equations are in essence the same. If, however, one allows for discontinuous (higher) derivatives of the contour line at the pole, the differently parameterized Euler Lagrange equations cease to be equivalent and describe different physical problems. It nevertheless appears to be true that the elastic energy corresponding to smooth contours remains a global minimum. ∗On leave from: Department of Theoretical Physics, J.Stefan Institute, P.O.Box 100, 61111 Ljubljana, Slovenia.