First, see whether it is arithmetic progression or geometric series;

Secondly, see if it is a simple function about natural numbers, such as power function, exponential function, logarithmic function and so on;

Thirdly, see if the difference between the front and rear numbers conforms to the characteristics of the above two items. If so, we can calculate the general formula according to the recursive formula.

Finally, see if there is a relationship between the two items, for example, the latter item is a function about the former item.

Try all the above, basically the same. Besides, Sandy doesn't know.

The code word is not easy, please adopt it.