ABSTRACT
The expression time series data, or time series, usually refers to a set of observations collected sequentially in time. These observations could have been collected at equally spaced time points. In this case we use the notation yt with (t = . . . ,−1, 0, 1, 2, . . .), i.e., the set of observations is indexed by t, the time at which each observation was taken. If the observations were not taken at equally spaced points then we use the notation yti , with i = 1, 2, . . ..
