Problem:
Let . Find the number of bijective functions for which there exists at least one such that
Solution
Solution:
Answer: 359108
We count the complement - the number of functions such that for all , .
The condition is equivalent to for all . If , the inequality is automatically satisfied for . Otherwise, if but , then we will have , allowing the inequality to be satisfied for , . Else, if , say and , then or . Thus the function allows us to partition the elements of into three groups:
(a) those such that ,
(b) those that form pairs such that and , and
(c) those that form quartets such that permutes them as ( ) or (i ), in cycle notation.
Let be the number of elements of the second type. Note that is even.
Case 1: There are no elements of the third type. If , there are possibilities. If , there are possibilities. If , there are possibilities. If , there are possibilities. If , there is 1 possibility. In total, case 1 offers possibilities.
Case 2: There are 4 elements of the third type. There are 21 ways to choose the quartet . For each way, there are two ways to assign the values of the function to each element (as described above). For the remaining 5 elements, we divide into cases according to the value of . If , there are possibilities. If , there are possibilities. If , there is one possibility. In total, case 2 offers possibilities.
Case 3: There are 8 elements of the third type. There are 5 ways to choose the unique element not of the third type. Of the remaining eight, there are 3 ways to divide them into two quartets, and for each quartet, there are 2 ways to assign values of . In total, case 3 offers possibilities.
Therefore, the number of functions such that for at least one , is .