#ref(http://www.codecogs.com/gif.latex?z_t = \displaystyle\sum^p_{j=1}\phi_jz_{t-j}+\sigma_\epsilon\xi_t) http://www.codecogs.com/png.latex?z_t = \displaystyle\sum^p_{j=1}\phi_jz_{t-j}+\sigma_\epsilon\xi_t.gif