A gauge-invariant chiral unitary framework for kaon photo- and electroproduction on the proton B. Borasoy, P. C. Bruns, U. -G. Meißner and R. Nißler Eur. Phys. J. A, 34 2 (2007) 161-183 Published online: 12 November 2007 DOI: 10.1140/epja/i2007-10492-4