Complex sentence: For small Ursell number, "U" ≪ 1, linear wave theory is applicable, otherwise (and most often) a non-linear theory, like the Korteweg--de Vries equation or Boussinesq equations have to be used.
Split into simpler sentences: For small Ursell number, "U" ≪ 1, linear wave theory is applicable. Otherwise (and most often) a non-linear theory, like the Korteweg--de Vries equation or Boussinesq equations has to be used.