在一些数学试题中常出现这种问题 :问题 1　设A ={ 1,2 ,3} ,f是A到A的映射 ,则满足 f(f(x) ) =x映射 f有多少个 ?问题 2　设A ={ 1,2 ,3} ,f是A到A的映射 ,则满足 f(f(x) ) =f(x)映射 f有多少个 ?对于上述问题 ,由于集合A中元素仅有三个 ,情况比较简单 ,只要通过列举便可获得 This problem often occurs in some math questions: Question 1 Let A = {1,2,3}, f be the mapping of A to A, then satisfy f(f(x)) =x How many are the map f? Question 2 Let A = { 1, 2, 3}, f be the mapping of A to A, then satisfy f(f(x)) = f(x) How many of the map f? For the above problem, since there are only three elements in set A One, the situation is relatively simple, as long as the list can be obtained