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I am doing some exercises in Algebra: Chapter 0. In the second chapter, we are asked to prove the following:

$G$ is a finite group with a unique element $f$ of order $2$. Then $\operatorname{\Pi_{g\in G}}g=f$.

This result is highly plausible. If we multiply the elements in the order of \begin{equation}e\cdot f\cdot \text{elements of order 3}\cdot\text{elements of order 4}\cdots,\end{equation} and pair elements with their inverses, then we get $f$, since it is the only element that does not have a couple.

However this is only one possible order of multiplication, and we know that in general different order give different results.

So I wonder how we can do the general case. Thanks!

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    Clearly, the group $G$ is from even order.2012-11-15
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    It's not true for arbitrary orders of the group elements. For example, in the quaternion group $Q_8$ the product can be either $f$ or the identity.2012-11-15
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    You need to change it to **finite abelian group**. See http://www.math.fsu.edu/~aluffi/algebraerrata/Errata.html (p.49)2012-11-15
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    @DerekHolt: In fact for any $g\in G$ there is a $n$ such that $g^n\in H=\langle f\rangle$. So the product of all elemnts would be $f$ or $e_G$. Right?2012-11-15
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    (The author even says: "My personal favorite is the missing abelian at p.49, Exercise 1.8.")2012-11-15
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    @Amr, yup its not possible if its not Abelian, marked my answer for deletion.2012-11-15
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    @BabakSorouh, that's the most I could prove passing to the quotient $\,G/\langle x\rangle\,$ , with $\,x\,$ the only involution. What's also true is that the product of all the elements in an *odd* order group is always contained in the group's derived subgroup.2012-11-15
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    It is not true that you have only one element without a pair - the identity element does not have a separate inverse either. Not important in this case as it doesn't affect the answer, but one to be aware of when counting elements (e.g. the argument that a p-group has non-trivial centre by examining the size of conjugacy classes).2012-11-15
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    @BabakSorouh, let $\,G=\{g_0:=1,g_1,...,g_n\}\,\,,\,n\,$ an even natural, be a group. In the quotient group $\,G/G'\,$ we have that the factors of the product $$\prod_{i=0}^n(g_iG')$$ are commutative, so we can pair each coset $\,g_iG'\,$ with $\,g_i^{-1}G\,$ and we thus get...2012-11-16
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    @BabakSorouh, knock yourself out. Thanks.2012-11-16

1 Answers 1

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In general we have a result about the order of elements that says $|aba^{-1}|=|b|$ for all $a,b\in G$.

Let $p=\left|\prod_{g\in G} g\right|$

Using this result, assuming $G$ is abelian, we can steadily "remove" pairs $(a,a^{-1})$ from $p$. However, $f$ is the only element which is its own inverse, so this process stops when we have $p=|f|=2$. But $f$ is the only element of order $2$ in $G$, so

$$\prod_{g\in G} g = f$$

  • 1
    You seem to be assuming $\,G\,$ is abelian, which is *not* given.2012-11-15
  • 0
    You have a word of elements of $G$ and cannot use that property without assuming $G$ is abelian, so I agree with Don.2012-11-15
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    Of course, you are right. I apologize.2012-11-15
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    The question is, do we have $|abcb^{-1}d| = |acd|$ in a non-abelian group? Because if we do, then your result holds; you can remove pairs $(b, b^{-1})$ from the word without changing the total order. I don't know, but by conjugation by $d$ above, we can rename $da$ to $a$ and explore whether $|abcb^{-1}| = |ac|$2012-11-15
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    It's clear that in any nonabelian group you can get at least two different products of all group elements, by using different orders.2012-11-15
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    Thanks! But for abelian groups it is trivial, and actually my problem already contains the idea of the proof.2012-11-15