Hasselmann, Klaus MPI for Meteorology, Max Planck Society;
1520-0485(1979)009_0373_avtfed_2.0.co (Publisher version), 644KB
Long, R. B., & Hasselmann, K. (1979). Variational technique for extracting directional spectra from multicomponent wave data. Journal of Physical Oceanography, 9, 373-381. doi:10.1175/1520-0485(1979)009<0373:AVTFED>2.0.CO;2.