The isotropic XY model on the inhomogeneous periodic chain
  The static and dynamic properties of the isotropic XY-model $(s=1/2)$ on the inhomogeneous periodic chain, composed of \emph{N} segments with \emph{n} different exchange interactions and magnetic moments, in a transverse field \emph{h} are obtained exactly at arbitrary temperatures. The properties are determined by introducing the generalized Jordan-Wigner transformation and by reducing the problem to a diagonalization of a finite matrix of \emph{n-th} order. The diagonalization procedure is discussed in detail and the critical behaviour induced by the transverse field, at T=0, is presented. The quantum transitions are determined by analyzing the behaviour of the induced magnetization, defined as $(1/n)\sum_{m=1}^{n}\mu_{m}<S_{j,m}^{z}>$ where $\mu_{m}$ is the magnetic moment at site \emph{m} within the segment \emph{j}, as a function of the field, and the critical fields determined exactly. The dynamic correlations, $<S_{j,m}^{z}(t)S_{j^{'},m^{'}}^{z}(0)>$, and the dynamic susceptibility $\chi_{q}^{zz}(\omega)$ are also obtained at arbitrary temperatures. Explicit results are also presented in the limit T=0, where the critical behaviour occurs, for the static susceptibility $\chi_{q}^{zz}(0)$ as a function of the transverse field \emph{h}, and for the frequency dependency of dynamic susceptibility $\chi_{q}^{zz}(\omega)$. Also in this limit, the transverse time-correlation $<S_{j,m}^{x}(t)S_{j^{'},m^{'}}^{x}(0)>$, the dynamic and isothermal susceptibilities, $\chi_{q}^{xx}(\omega)$ and $\chi_{T}^{xx}$, are obtained for the transverse field greater or equal than the saturation field. 
J. P de Lima, T. F. A. Alves, L. L. Goncalves