Transition to a boundary-value problem in the Ising model

The Ising problem is regarded as a boundary-value problem for the free energy function in a space whose variables are the field and the coupling constant. This approach reduces the number of approximations, and the existing approximate methods may therefore be improved. For example, the quasiehemteal method is derived independently of the average-field modeI and is rendered sensitive to the lattice symmetry. This does not happen at the cost of the advantages of the quasiehemical method (for example, the short-range order is still allowed for accurately).