This week, in the MAT8181 Time Series course, we’ve discussed unit root tests. According to Wold’s theorem, if is (weakly) stationnary then where is the innovation process, and where is some deterministic series (just to get a result as general as possible). Observe that as discussed in a previous post. To go one step further, there is also the Beveridge-Nelson decomposition : an integrated of order one process, defined as…