find sufficient statistics for theta.

Let X1, ..., Xn be observations satisfying Xi = 0Xi-1 + Zi, i = 2, ..., n, where X1, Z2, . .., Zn are independent with X1 ~ N(0, 1) and Zi ~ N(0, 1), i = 2, ...,n. The parameter space is 0 = {0 : -1