A quantitative study of the Kosterlitz-Thouless phase transition in a   system of two-dimensional plane rotators ( XY model ) by high temperature   expansions through $\beta^{20}$
  High temperature series expansions of the spin-spin correlation function for the plane rotator (or XY) model on the square lattice are extended by three terms through order $\beta^{20}$. Tables of the expansion coefficients are reported for the correlation function spherical moments of order $l=0,1,2$. The expansion coefficients through $\beta^{15}$ for the vorticity are also tabulated. Our analysis of the series supports the Kosterlitz-Thouless predictions on the structure of the critical singularities and leads to fairly accurate estimates of the critical parameters. 
P. Butera and M. Comi (Istituto Nazionale di Fisica Nucleare   Dipartimento di Fisica, Universit\`a di Milano Via Celoria 16...