M.Campanino, D.Ioffe, Y.Velenik
Ornstein-Zernike Theory for the finite range Ising models above T_c
(312K, latex2e with 5 PS figures)

ABSTRACT.  We derive precise Ornstein-Zernike asymptotic formula for the decay 
of the two-point function in the general context of finite range Ising 
type models on Z^d. 
The proof relies in an essential way on the a-priori knowledge of the 
strict exponential decay of the two-point function and, 
by the sharp characterization of phase transition due to Aizenman, 
Barsky and Fernandez, goes through in the whole of the high temperature 
region T > T_c. 
As a byproduct we obtain that for every T > T_c, the inverse correlation 
length is an analytic and strictly convex function of direction.