Do we need to be able to state/ write the formula for different time series models or would that be given in the question?
It's been asked before, definition of stationarity i don't see why they can't ask for definition of ARIMA or GARCH, so yep....know the formulas
I'd agree with Edwin...you should know the general forms of the models, how they might be used and how they might be fitted/parametrised etc
