The first floor is so strong, such a transformation can make you think that the solution is right. I'm just adding something here. If the value of A 1 is not given in the title, then discuss the value of A 1 first. When A 1=0, plus or minus 1, a [n] = a1; When A 1 does not take these three values, this problem can be solved by the idea of function iteration and fixed point, which should also be the general solution of this kind of recursive formula. I have calculated that the answer is the same as that on the first floor, so I won't write down the specific steps. It should be said that there is a simple recursive formula for this kind of problem, but the answer is generally quite complicated. If you are interested, you can check it out: Chapter 2, Section 5 of Function Iteration of Mathematical Olympiad Course published by Hunan Normal University Press. Mastering this kind of thinking will be a great weapon to deal with this kind of problem. ^_^