Cnoidal wave is the wave height, "λ" is the wavelength, "c" is the phase speed and "η" is the trough elevation. Further cn is one of the Jacobi elliptic functions and "K"("m") is the complete elliptic integral of the first kind; both are dependent on the elliptic parameter "m". The latter, "m", determines the shape of the cnoidal wave. For "m" equal to zero the cnoidal wave becomes a cosine function, while for values close to one the cnoidal wave gets peaked crests and (very) flat troughs. For values of "m" less than 0.95, the cnoidal function can be approximated with trigonometric
Zero sound transport equation is valid in the regime where formula_36 and formula_37. In many systems, formula_38 only slowly depends on the angle between formula_39 and formula_40. If formula_29 is an angle-independent constant formula_42 with formula_43 (note that this constraint is stricter than the Pomeranchuk instability) then the wave has the form formula_44 and dispersion relation formula_45 where formula_46 is the ratio of zero sound phase velocity to Fermi velocity. If the first two Legendre components of the Landau parameter are significant, formula_47 and formula_48, the system also admits an asymmetric zero sound wave solution formula_49 (where formula_50 and formula_51 are the