The critical equation of state of the two-dimensional Ising model

We compute the 2n-point coupling constants in the high-temperature phase of the two-dimensional Ising model by using transfer-matrix techniques. This provides the first few terms of the expansion of the effective potential (H elmholtz free energy) and of the equation of state in terms of the renormal ized magnetization. By means of a suitable parametric representation, we de termine an analytic extension of these expansions, providing the equation o f state in the whole critical region in the t, h plane.