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Algebra Difficulty 8.5 Shortlist Prove it Belarus

Let n3n \ge 3 be a positive integer. Positive integers are written in two rows on the whiteboard a1 a2  an b1 b2  bna_1\ a_2\ \dots\ a_n\ b_1\ b_2\ \dots\ b_n, where the sets a1,a2,,ana_1, a_2, \dots, a_n and b1,b2,,bnb_1, b_2, \dots, b_n are some permutations of the numbers 1,2,,n1, 2, \dots, n. The teacher allows the student Dima to use the following two operations as many times as he likes and in any order:
(A) replace each number aa of the first line with the number that is in the aa'th place (from left to right) of the second line;
(B) replace each number bb of the second line with the number that is in the bb'th place (from left to right) of the first line.

Thus, for example, the operation AA transforms the table
123231 \begin{array}{ccc} 1 & 2 & 3 \\ 2 & 3 & 1 \end{array}
into the table
231231 \begin{array}{ccc} 2 & 3 & 1 \\ 2 & 3 & 1 \end{array}
, which in turn is transformed by the operation BB into the table
231312 \begin{array}{ccc} 2 & 3 & 1 \\ 3 & 1 & 2 \end{array}

Can Dima obtain the table
2314n1n2345n1 \begin{array}{ccccccc} 2 & 3 & 1 & 4 & \dots & n-1 & n \\ 2 & 3 & 4 & 5 & \dots & n & 1 \end{array}
from the table
2134n1n2345n1 \begin{array}{ccccccc} 2 & 1 & 3 & 4 & \dots & n-1 & n \\ 2 & 3 & 4 & 5 & \dots & n & 1 \end{array}

Solution

Answer: no. Each table a1b1a2b2anbn\begin{array}{c} a_1 \\ b_1 \end{array} \begin{array}{c} a_2 \\ b_2 \end{array} \dots \begin{array}{c} a_n \\ b_n \end{array} corresponds to a pair of permutations (s,t)Sn×Sn(s,t) \in S_n \times S_n such that
s=(12na1a2an)andt=(12nb1b2bn). s = \begin{pmatrix} 1 & 2 & \dots & n \\ a_1 & a_2 & \dots & a_n \end{pmatrix} \quad \text{and} \quad t = \begin{pmatrix} 1 & 2 & \dots & n \\ b_1 & b_2 & \dots & b_n \end{pmatrix}.
The operation AA transforms a pair (s,t)(s, t) into the pair (st,t)(st, t), where the composition of the permutations stst is read from left to right – first the permutation ss is applied, and then the permutation tt. Accordingly, the operation BB transforms a pair (s,t)(s, t) into the pair (s,ts)(s, ts).
For each pair of permutations (s,t)(s,t) we can consider the expression sts1t1sts^{-1}t^{-1}, which is called the commutator of this pair and is denoted by [s,t][s,t]. Let us calculate the commutators of the pairs (st,t)(st,t) and (s,ts)(s,ts):
[st,t]=(st)t(st)1t1=sttt1s1t1=[s,t]and[s,ts]=s(ts)s1(ts)1=stss1s1t1=[s,t]. [st, t] = (st)t(st)^{-1}t^{-1} = sttt^{-1}s^{-1}t^{-1} = [s, t] \quad \text{and} \quad [s, ts] = s(ts)s^{-1}(ts)^{-1} = stss^{-1}s^{-1}t^{-1} = [s, t].
Thus, the commutator is invariant under the operations A and B.

The table
2134n1n2345n1 \begin{array}{|c|ccccccc|} \hline 2 & 1 & 3 & 4 & \dots & n-1 & n \\ \hline 2 & 3 & 4 & 5 & \dots & n & 1 \\ \hline \end{array}
corresponds to a pair of permutations (1 2)(1\ 2) and (1 2  n)(1\ 2\ \dots\ n),
which are cycles, and the table
2314n1n2345n1 \begin{array}{|c|ccccccc|} \hline 2 & 3 & 1 & 4 & \dots & n-1 & n \\ \hline 2 & 3 & 4 & 5 & \dots & n & 1 \\ \hline \end{array}
corresponds to a pair of permutations
(1 2 3)(1\ 2\ 3) and (1 2  n)(1\ 2\ \dots\ n), which are also cycles. Their commutators are easy to find:
[(1 2),(1 2  n)]=(1 2)(1 2  n)(2 1)(n  2 1)=(1 2 n), [(1\ 2), (1\ 2\ \dots\ n)] = (1\ 2)(1\ 2\ \dots\ n)(2\ 1)(n\ \dots\ 2\ 1) = (1\ 2\ n),
[(1 2 3),(1 2  n)]=(1 2 3)(1 2  n)(3 2 1)(n  2 1)=(1 2 3 n). [(1\ 2\ 3), (1\ 2\ \dots\ n)] = (1\ 2\ 3)(1\ 2\ \dots\ n)(3\ 2\ 1)(n\ \dots\ 2\ 1) = (1\ 2\ 3\ n).
Since they are not equal, Dima will not be able to get the second table from the first one.

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