where |a1| < 1 must be true for stationarity. 

The PsiAR1() time series function produces a first order autoregressive time series with mean mean, volatility volatility, autoregressive coefficient coef1 and time value val0. 

This is a well known and used time series due to its simplicity and it's ability to provide a good fit to data.  An AR(1) time series is depicted by an autocorrelation function (ACF) that decreases geometrically and a partial autocorrelation function (PACF) that reduces to 0 after lag 1. 

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