The entropy of a system of n spins with magnetic moment m, where 2s is the spin excess (n is approximately sigma = ln[g(n, s=0)] - 2s^2/n, for small s and large n. given that the energy of the state with spin excess 2s in a magnetic field b is = -2smb, calculate the average value of the fractional magnetisation, 2(s)/n, as a function of temperature.