Partition function for the two-dimensional square lattice Ising model in   a non-zero magnetic field-A heuristic analysis
  The exact partition function of the two-dimensional nearest neighbour Ising model pertaining to square lattices is derived for N sites in the case of a non-vanishing magnetic field.When the magnetic field is zero,the partition functions estimated from the present analysis are identical with those arising from Onsager's exact solution. 
G. Nandhini and M. V. Sangaranarayanan