J. Blau, K. R. Cohn, and W.B. Colson,
   "Four-Dimensional Models of FEL Amplifiers and Oscillators",
   in Proc. 37th International Free Electron Laser Conference, Daejeon, Republic of Korea, August 2015, paper WEP008, pp. 607-614,
    ISBN: 978-3-95450-134-2,
    doi:10.18429/JACoW-FEL2015-WEP008,
    http://accelconf.web.cern.ch/AccelConf/FEL2015/papers/wep008.pdf, 2015.