A. Alastuey et F. Cornu, CRITICAL LINE NEAR THE ZERO-DENSITY CRITICAL-POINT OF THE KOSTERLITZ-THOULESS TRANSITION, Journal of statistical physics, 87(3-4), 1997, pp. 891-895
Using low-fugacity expansions, we study the exact behavior of the Kost
erlitz-Thouless critical line near the zero-density critical point. We
show that the critical temperature deviates from its zero-density val
ue by a term proportional to the square root of the density.