Spin-correlation functions and susceptibilities in the easy-plane XXZ chain

We present a Green's-function theory of magnetic short-range order in the S = 1/2 easy-plane XXZ chain based on the projection method for the dynamic spin susceptibility and a decoupling of three-spin operator products introd ucing vertex parameters. The longitudinal and transverse static susceptibil ities and two-point correlation functions of arbitrary range are calculated self-consistently for all wave numbers, temperatures, and anisotropy param eters - 1 less than or equal to Delta less than or equal to 1. In the easy- plane ferromagnetic region (Delta < 0), the longitudinal correlators of spi ns at distance n change sign at a finite temperature T-0(n,<Delta>), in rea sonable agreement with recent data obtained by finite-chain diagonalization s. The temperature dependence of the uniform static susceptibilities exhibi ts a maximum which is explained as an effect of magnetic shea-range order w hich decreases with increasing temperature.