is time series a linear model what is linear time series is a time series model linear is an arma model linear is the model x_t linear is the time series a linear time series which of the following models is not linear what is the linear time series tsay is arma a linear time series is a function of time a linear time series model? why are linear time series model not linear is $d(t) linear are financial time series linear is $var linear what is linear time series is x_t a linear time series are tsay time series linear is x_t linear? is x_t a linear time series is xt linear time series