I would have a question concerning the modeling of a non-stationary time series. I do know some of the models for stationary time series such as AR MA ARMA ARCH or GARCH, but what if the time series is non-stationary. I know that it is possible to make it stationary by differencing, but maybe there would be a way to do it differently.
My series is kind of constant (it represents a baseline) but follows three trends/patterns : - First, increasing - Second, kind of constant - And third, decreasing
Would you have an idea of how to represent it elegantly via a time series (or others) ?
Thanks in advance !