[American Institute of Aeronautics and Astronautics 19th AIAA, Fluid Dynamics, Plasma Dynamics, and Lasers Conference - Honolulu,HI,U.S.A. (08 June 1987 - 10 June 1987)] 19th AIAA, Fluid Dynamics, Plasma Dynamics, and Lasers Conference - Comparison of sideband spectra obtained from analytical formula and from numerical simulation in free-electron laser