Prove that exist $x\in \left\{ 1,…,14 \right\}$ such that $\sigma(x)=x$, where $\sigma\in S_{14}$ and $|\sigma|=28$

group-theorypermutations

Let $\sigma\in S_{14}$ which is an even permutation of the order of $28$.
Prove that exist $x\in \left\{ 1,…,14 \right\}$ such that $\sigma(x)=x$.

My try:

We know that the permutation order is equal to the least common multiple of cycles that make up a given permutation and $28=2\cdot2\cdot7$.

So $\sigma$ must be character $(a_1 a_2 a_3 a_4)(b_1 b_2 … b_7)$ – composition of row cycle $4$ and row cycle $7$
because if $\sigma$ would be a character $(a_1
,a_2)(b_1b_2)(c_1…c_7)$
then $|\sigma|=2\cdot7=14$ which is contrary to the assumption.

That's why $4+7=11$ elements elements undergo nontrivial permutations and $14-11=3$ elements pass on to each other.

So $\sigma$ has a character:

$$\sigma=\begin{pmatrix} a_1 & a_2 & a_3 & a_4 & b_1 & b_2 & b_3 & b_4 & b_5 & b_6 & b_7 & c_1 & c_2 & c_3 \\ a_2 & a_3 & a_4 & a_1 & b_2 & b_3 & b_4 & b_5 & b_6 & b_7 & b_1 & c_1 & c_2 & c_3\end{pmatrix}$$

Moreover we have information that $\sigma=(a_1 a_2 a_3 a_4)(b_1 b_2 … b_7)$ is composition an even number of transpositions.

However these are my only thoughts and I don't know what to do next to come to the thesis.

EDIT:

According to the remark from @EricTowers $\sigma$ can still have a character $(a_1 a_2 a_3 a_4)(b_1 b_2 … b_7)(c_1c_2)$ then $$\sigma=\begin{pmatrix} a_1 & a_2 & a_3 & a_4 & b_1 & b_2 & b_3 & b_4 & b_5 & b_6 & b_7 & c_1 & c_2 & c_3 \\ a_2 & a_3 & a_4 & a_1 & b_2 & b_3 & b_4 & b_5 & b_6 & b_7 & b_1 & c_2 & c_1 & c_3\end{pmatrix}$$

Best Answer

Let $n_k$ be the number of $k$-cycles in the disjoint cycle decomposition of $\sigma$. Then

  • $k \in \{1,2,4,7,14,28\}$

  • $n_{28}=0$ because $28>14$

  • $n_{14}=0$ because a $14$-cycle is not even

  • $n_4 \ge 1$

  • $n_7 \ge 1$

  • $n_1 + 2n_2 + 4n_4 +7n_7 = 14$

The last equation has no solutions if $n_1=0$. Thus $n_1\ge1$, as required.

Actually, the only solutions are $(n_1,n_2,n_4,n_7)=(1,1,1,1)$ and $(3,0,1,1)$. However, since $\sigma$ is even, we must have $n_2+n_4$ even and so the only solution is $(n_1,n_2,n_4,n_7)=(1,1,1,1)$.

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