The order of an ARIMA model is usually denoted by the notation ARIMA(p,d,q), where Thus, when an autoregressive operator ... ith input series is the numerator polynomial of the transfer function for ...

This is called function notation and is just another way of expressing the relationship between two variables. Therefore, in general \(y = f(x)\).

Composite functions are made when the output from one function is used as the input of another function. The names of the functions are written next to each other, with the function that is used ...