Korteweg–de Vries equation
For long waves ("λ" ≫ "h") with small Ursell number, "U" ≪ 32 π2 / 3 ≈ 100, linear wave theory is applicable. Otherwise (and most often) a non-linear theory for fairly long waves ("λ" &gt; 7 "h") – like the Korteweg–de Vries equation or Boussinesq equations – has to be used.