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I'm currently studying for a theory exam in Abstract Algebra. One question is about first defining the concepts of even and odd permutations and then explain why these concepts are well-defined. So, I find this a bit difficult and I'm wondering if you guys can help me out?

Why are Even and Odd permutations well-defined?

Thank You

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marked as duplicate by José Carlos Santos, YuiTo Cheng, Dietrich Burde abstract-algebra Jun 16 at 11:53

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The definitions of even and odd that I'm familiar with have to do with expressing a permutation as a product of transpositions.

Specifically, every permutation $\sigma$ can be expressed as a product of transpositions $\tau_1 \tau_2 \cdots \tau_n$. Then $\sigma$ is even if $n$ is even, and $\sigma$ is odd if $n$ is odd, where $n$ is the number of transpositions appearing in the product.

The problem is that there are lots of ways of expressing a permutation as a product of transpositions. For example $$(1~~2~~3) = (1~~3)(1~~2) = (2~~1)(2~~3) = (1~~2)(1~~3)(2~~3)(2~~1)$$ So you need to show that an even permutation $\sigma$ doesn't magically become odd just by expressing it in a different way as a product of transpositions—that is, you need to show that if some expression of $\sigma$ as a product of transpositions has an even (resp. odd) number of transpositions in the product, then all such expressions do.

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  • $\begingroup$ Ok! This is good. Thanks! $\endgroup$ – Victor Galeano Jun 17 at 8:12

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