I'm wondering how to detect the existence of more than one unit root. I've tested the non-stationnarity of my serie using the strategy of test of Dickey-Fuller (joint hypothesis: Fisher), and I detect unit root. however, after differencing the serie, I think that it's still non stationary and it has another unit root. My questions are:
How can I detect more than one unit root
Am i obliged to apply the strategy of Fisher of Dickey-fuller again to the differienced serie or since I did not detect a trend in the level serie I'll not detect it in the differencing one?