operators with magnetic fields: factorisation method on curved surfaces", J. Math. Phys. 42 (2001) 590, [arXiv:0007034[math- ph]] [94] P. Goddard, J. Goldstone, C. Rebbi and C. B. Thorn, "Quantum dynamics of a massless relativistic string", Nucl. Phys. B 56 (1973) 109. [95] I. V. Gorbunov, S. M. Kuzenko an