Two permutations and
of the numbers
are said to intersect if for some value of in the
range . Show that there exist permutations
of the numbers such that any other such
permutation is guaranteed to intersect at least one of these
permutations.
Solution
Let be a positive integer. Let be the smallest positive integer with
. Since , . Let
be the set of positive integers from to . Let ,
be .
Let be the set of of permutations of .
Let be the set of cyclic permutations of , there are
cyclic permutations in all, and acts transitively on , i.e.
for every pair of elements , there is an element of
that maps to .
Let be the permutations in that leave
fixed, and restricted to yield one of the permutations in .
There is a natural one-to-one correspondence between and .
We claim that the permutations intersect every permutation in
.
Suppose, to the contrary, that there exists a permutation
that does not intersect any permutation in . Since acts
transitively on the permutation cannot send any element of
to any other element of , therefore it must send all the
elements of to , but since has elements and , this is impossible
by the pigeonhole principle. Therefore such a permutation cannot
exist, and the permutations in intersect every permutation in
.
For we get , which is the required special
case of the general result above.