  Fisher-Hartwig formula has been successful applied to describe the von
Neumann and R\'enyi entropies of a block of spins in the ground state of XX
spin chain. It was based on a determinant representation. In this paper, we
generalize the free fermion method to obtain an exact formulation for the
entropy of any finite subsystem in XX spin chain. Based on this, we derive a
determinant representation of the entropy of multiple disjoint intervals in the
ground state of $XX$ model.
