Next-nearest-neighbour correlation functions of the spin-1/2 XXZ chain at the critical region

The correlation functions of the spin-1/2 XXZ chain in the ground state are expressed in the form of multiple integrals. For -1 < Delta < 1, they were obtained by Jimbo and Miwa in 1996. In particular, the next-nearest-neighbour correlation functions are given as certain three-dimensional integrals. We show that these can be reduced to one-dimensional integrals and thereby we evaluate the values of the next-nearest-neighbour correlation functions. We have also found that the remaining one-dimensional integrals can be evaluated analytically, when v = cos(-1) (Delta)/pi is a rational number.