Further extensions of the high-temperature expansions for the   two-dimensional classical XY model on the triangular and the square lattices
  The high-temperature expansions for the spin-spin correlation function of the two-dimensional classical XY (planar rotator) model are extended by two terms, from order 24 through order 26, in the case of the square lattice, and by five terms, from order 15 through order 20, in the case of the triangular lattice. The data are analyzed to improve the current estimates of the critical parameters of the models. 
P. Butera and M. Pernici