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