Question #339125

How many bijections (bijective functions) are there from A to A with the

property that no element of A is mapped to itself? (A is a set with 5 elements)


1
Expert's answer
2022-05-10T13:27:43-0400

There are 5!5! different bijections from AA to AA. Namely the first element can be mapped into 55 different elements. The second one into 44 elements and so on. Using a multiplication principle of combinatorics, we receive: 5!=1205!=120 different bijections. We point out that a bijection that has 44 elements mapped to itself and 55 elements mapped to itself is the same. There is 11 bijection of this type. Consider bijections that have 33 elements mapped to itself, but not 44 and 55 . There are C53=5!3!2!=10C_5^3=\frac{5!}{3!2!}=10 maps of this type. Consider maps that have only 22 elements mapped to itself. There are 2C52=25!3!2!=202C_5^2=2\frac{5!}{3!2!}=20 maps of this type. Consider maps that have 11 element mapped to itself. As an example, we fix 11. There are 4!4! different bijections that have 11 mapped to itself. They contain 11 bijection that has 44 and 55 elements mapped to itself, C42=4!2!2!=6C_4^2=\frac{4!}{2!2!}=6 bijections that have 33 elements mapped to itself and 2C41=82C_4^1=8 bijections that have 22 elements mapped to itself. We receive: 4!168=94!-1-6-8=9 bijections. Thus, there are 99 bijections that have only 11 mapped to itself. We receive:4545 bijections that have only 11 element mapped to itself. Thus, we get: 1+10+20+45=761+10+20+45=76 different bijections that have at least 11 element mapped to itself. We receive: 12076=44120-76=44 bijections that have no elements mapped to itself.

Answer: there are 4444 bijections that have no elements mapped to itself.


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