what is stationarity of a time series when using time series, if the mean is nonzero, the mean should be when a time series is stationary, why is the mean constant for time series how is time series determined? how to test for stationarity what is the non zero mean for a time series why is the mean constant on the time series does $x_t$ have a non zero value what is the mean of nonzero time series how to test stationary time series what is the value of a non zero mean when do we find out if a series is stationary test time series for stationarity how to test a time series what is the mean of a time series what means stationary how to test stationarity in a time series how to test time series x-t for stationarity when is mu constant