Compton upconversion of twisted photons: backscattering of particles with non-planar wave functions U.D. Jentschura and V.G. Serbo Eur. Phys. J. C, 71 3 (2011) 1571 Published online: 01 March 2011 DOI: 10.1140/epjc/s10052-011-1571-z