Because of the cusp in u, the chemical potential du/dns has a discontinuity _μ = ¯hωc at integer ν. At these values of the filling factor an infinitesimal increase in electron density costs a finite amount of energy, so the electron gas can be said to be incompressible. The cusp in u at fractional ν exists because of the Coulomb interaction. The discontinuity _μ is now approximately _μ ≈ e2/ǫlm ∝ √B, which at a typical field of 6T in GaAs is 10meV, of the same magnitude as the Landau level separation ¯hωc ∝ B.