Problem:
Let be the set . A perfectutation is a bijective function from to itself such that there exists an such that , and that for any pair of integers and such that , there exists a positive integer such that . Let be the number of ordered pairs of perfectutations such that for all , but . Find the remainder when is divided by 2011.
Solution
Solution:
Answer: 2
Note that both and , when written in cycle notation, must contain exactly one cycle that contains more than 1 element. Assume has fixed points, and that the other elements form a cycle, (of which there are ways).
Then note that if fixes then implies fixes . So must send fixed points of to fixed points of . It must, therefore, send non-fixed points to non-fixed points. This partitions into two sets, at least one of which must be fixed by , since is a perfectutation.
If fixes all of the non-fixed points of , then, since any function commutes with the identity, fixes some of the fixed points and cycles the rest in ways. So there are choices, which is .
If fixes all of the fixed points of , then order the non-fixed points of such that . If then thus . Therefore the choice of uniquely determines for the rest of the , and . But has to be a perfectutation, so cycles through all the non-fixed points of , which happens if and only if is relatively prime to . So there are choices.
Therefore for any there are choices of , but one of them will be , which we cannot have by the problem statement. So there are options.
Now note that a permutation can not fix all but one element. So
Modulo 2011 (which is prime), note that all terms in the summand except the one where vanish. Thus, by Wilson's Theorem.