A threshold AR(1) process with boundary width 2δ>0 was defined by Brockwell and Hyndman in terms of the unique strong solution of a stochastic differntial equation whose coefficients are piecewise linear and Lipschitz. The positive boundary-width is a convenient mathematical device to smooth out the coefficient changes at the boundry and hence to ensure the existence and uniqueness of the strong solution of the stochastic differential equation from which the process is derived. In this paper the authors give a direct definition of a threshold AR(1) process with δ=0 in terms of the weak solution of a certain stochastic differential equation. Two characterizations of the distributions of the process are investigated. Both express the characteristic function of the transition probability distribution as an explicit functional of standard Brownian motion. It is shown that the joint distributions of this solution with δ=0 are the weak limits as δ↓0 of the distributions of the solution with δ>0. The sense in which an approximating sequence of processes used by Brockwell and Hyndman converges to this weak solution is also investigated. Some numerical examples illustrate the value of the latter approximation in comparison with the more direct representation of the process obtained from the Cameron-Martin-Girsanov formula and results of Engelbert and Schmidt. The authors also derive the stationary distribution (under appropriate assumptions) and investigate stability of these processes.
