2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/57905The two-dimensional Ising model is representable as a lattice free-fermion field theory in terms of the integral over anticommuting Grassmann variables. The exact solution in a zero magnetic field then follows by evaluating Gaussian fermionic integral for partition function. It is argued that by switching on of a non-zero magnetic field the non-local and non-Gaussian fermionic terms appear in the action, which rather imply approximating methods of the analysis.LaTeX, 6 pages, no figuresMathematical PhysicsStatistical MechanicsHigh Energy Physics - LatticeHigh Energy Physics - Theory82B20; 82B10; 82B26text