Question #203254

Let τ be a fixed odd permutation in . S₁₀ Show that every odd permutation in S₁₀ is

a product of τ and some permutation in A₁₀


1
Expert's answer
2021-06-07T13:38:03-0400

Solution:

Proof. Let σ\sigma^{\prime} be an odd permutation in SnS_{n} . We must show that there exists an even permutation μAn\mu \in A_{n} such that σ=σμ\sigma^{\prime}=\sigma \mu . Indeed, we may take μ=σ1σ\mu=\sigma^{-1} \sigma^{\prime} , since as the product of two odd permutations, it is an even permutation, and

σ=σ(σ1σ)\sigma^{\prime}=\sigma\left(\sigma^{-1} \sigma^{\prime}\right)

For completeness, let's prove directly that σ1σ\sigma^{-1} \sigma^{\prime} is even. From the definition of an odd permutation, there exist a finite number of transpositions τ1,,τm\tau_{1}, \ldots, \tau_{m} for some odd mNm \in \mathbb{N} such that

σ=τ1τm\sigma=\tau_{1} \ldots \tau_{m}

Similarly, since σ\sigma^{\prime} is also an odd permutation, there exist a finite number of transpositions τ1,,τ\tau_{1}^{\prime}, \ldots, \tau_{\ell}^{\prime} for some odd N\ell \in \mathbb{N} such that σ=τ1τ\sigma^{\prime}=\tau_{1}^{\prime} \ldots \tau_{\ell}^{\prime} . Consider now the permutation

μ=σ1σ\mu=\sigma^{-1} \sigma^{\prime}

I claim that this lies in AnA_{n} . Indeed we have

μ=σ1σ=τmτ1τ1τm+\mu=\sigma^{-1} \sigma^{\prime}=\underbrace{\tau_{m} \ldots \tau_{1} \tau_{1}^{\prime} \ldots \tau_{\ell}^{\prime}}_{m+\ell}

The sum of two odd numbers is even, and so it follows that this is an even permutation.

Thus, if we take n=10n=10 , we can prove it same way.


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