The stationarity conditions for an autoregressive (AR) process in general are reduced to a remarkably simple inequality if the lag coefficients are restricted to be identical. The condition is not only analytically elegant but also applicable in checking the validity of the stationarity conditions for such a restricted AR process of any order.
Im, Eric Iksoon; Hammes, David L.; and Wills, Doug, "Stationarity Condition for AR Index Process" (2006). Business Publications. 1.