A thermal form factor series for the longitudinal two-point function of the Heisenberg–Ising chain in the antiferromagnetic massive regime

We consider the longitudinal dynamical two-point function of XXZ quantum spin chain in antiferromagnetic massive regime. It has a series representation based on form factors transfer matrix model. The nth summand is multiple integral accounting for all n-particle–n-hole excitations matrix. In previous works, expressions factor amplitudes appearing under integrals were either again represented as or terms Fredholm determinants. Here, we obtain which reduces, zero-temperature limit, essentially to product two determinants finite matrices whose entries are known special functions. This will facilitate further analysis correlation function.