We now want to study the stability of the system with respect to small fluctuations in the concentration c , for example a sine wave of amplitude a and wavevector q = 2 π π / λ λ , for λ λ the wavelength of the concentration wave. To be thermodynamically stable, the free energy change δ δ F due to any small amplitude concentration fluctuation δ δ c = a sin ⁡ ⁡ ( q → → . r → → ) , must be positive.