# Prove that $(a_1a_2\cdots a_n)^{2} = e$ in a finite Abelian group

Let $G$ be a finite abelian group, $G = \{e, a_{1}, a_{2}, ..., a_{n} \}$. Prove that $(a_{1}a_{2}\cdot \cdot \cdot a_{n})^{2} = e$.

I've been stuck on this problem for quite some time. Could someone give me a hint?

• every element has an inverse, and you can reorder terms
– RHP
Jul 22, 2011 at 4:50
• Can I assume $(a_{1}a_{2} \cdot \cdot \cdot a_{n})^{2} = e$ is true? If I can, then $(a_{1}a_{2} \cdot \cdot \cdot a_{n})(a_{1}a_{2} \cdot \cdot \cdot a_{n}) = e$. If $ab = e$, then $b = a^{-1}$. So $(a_{1}a_{2} \cdot \cdot \cdot a_{n}) = (a_{1}a_{2} \cdot \cdot \cdot a_{n})^{-1}$. Then $(a_{1}a_{2} \cdot \cdot \cdot a_{n})(a_{1}a_{2} \cdot \cdot \cdot a_{n})^{-1} = e$. Thus $e = e$??? Jul 22, 2011 at 4:59
• @Jon: You cannot assume that what you are trying to prove is true; that leads to a circular argument. Jul 22, 2011 at 5:00
• @ Arturo: Thanks for the clear up :) Jul 22, 2011 at 5:12

The map $\phi:x\in G\mapsto x^{-1}\in G$ is an automorphism of $G$ so, in particular, it induces a bijection $G\setminus\{e\}\to G\setminus\{e\}$. It maps $b=a_1\cdots a_n$ to itself, so that $b=b^{-1}$ and, therefore, $b^2=e$.

• Automorphism? What's an automorphism? Well, you know, and I know, but do you reckon OP knows? Jul 22, 2011 at 5:57
• @Gerry: Not yet, but hopefully I'll read about automorphisms soon enough :) Jul 22, 2011 at 6:04
• @Gerry: he'll know soon enough :) Jul 22, 2011 at 6:05
• This is very clever and succinct, +1! Jul 22, 2011 at 7:22

Here is a hint: for any given $a_i\in G$, there are two possibilities: either

• $a_i$ is its own inverse, or
• $a_i$ is not its own inverse, but rather $a_j=a_i^{-1}$ for some $j\neq i$.
• So for the first case where $a_{i}$ = $a_{i}^{-1}$. $(a_{1}a_{2} \cdot \cdot \cdot a_{n})^{2} = (a_{1}a_{2} \cdot \cdot \cdot a_{n})(a_{1}a_{2} \cdot \cdot \cdot a_{n}) = (a_{1}a_{2} \cdot \cdot \cdot a_{n})(a_{1}^{-1}a_{2}^{-1} \cdot \cdot \cdot a_{n}^{-1}) = a_{1}a_{1}^{-1} \cdot \cdot \cdot a_{n}a_{n}^{-1} = e$ Jul 22, 2011 at 5:09
• No, that reasoning isn't correct; for example, it might be the case that $a_3=a_3^{-1}$, but $a_4=a_5^{-1}$ and $a_5=a_4^{-1}$. In other words, the two cases I described above concern any single given $a_i\in G$, but there is no reason that every $a_i\in G$ should fall into the same case. Jul 22, 2011 at 5:14
• To explain it in yet another manner, it is false that either $$\bullet\quad\text{For every }a_i\in G,\,\,a_i\text{ is its own inverse.}$$ or $$\bullet\quad\text{For every }a_i\in G,\,\,a_i\text{ is not its own inverse.}$$ There may be some mixture. Combine the two arguments you have thought of for each case, and you will have an argument that will work in general. Jul 22, 2011 at 5:22
• $(a_{1}a_{2} \cdot \cdot \cdot a_{n})(a_{1}a_{2} \cdot \cdot \cdot a_{n})$. Then each $a_{i}$ in the second set of parentheses can be replaced appropriately by $a_{i}^{-1}$ if $a_{i}$ is its own inverse or by $a_{i}^{-1} = a_{j}$ for some $j \neq i$. Then since $G$ is Abelian, $a_{1}a_{1}^{-1} \cdot \cdot \cdot a_{n}a_{n}^{-1} = e$. Is this better? Jul 22, 2011 at 5:28
• This is precisely the right idea! However, there is one small detail you should also prove to make a fully rigorous argument: you should show that an element and its inverse are paired uniquely. Supposing it were possible that $a_k=a_i^{-1}$ and $a_k=a_j^{-1}$ for $i\neq j$, then after rewriting the terms in the second set of parentheses, we'd have two copies of $a_k$, and be missing some other element of $G$. However, this cannot happen. Do you see why? Jul 22, 2011 at 5:31

Note that a question which is in some sense more natural is: "What is the product of all the elements of a finite abelian group?" The given question gives some information about this product, namely that it is an element of order at most $2$.

The answer to the more general question is given by the following result.

Wilson's Theorem in a Finite Abelian Group: Let $G$ be a finite abelian group. Then the product of all elements of $G$ is equal to the identity unless $G$ has exactly one element $t$ of order $2$, in which case the product is $t$. For a proof see e.g. $\S 6$ of these notes.

In fact one can view the answer to the OP's question as 2/3 of the way towards a proof of WTFAG. Namely, we take the product of all of the elements in the group, and note that the elements which have order greater than $2$ occur in pairs $x,x^{-1}$. Therefore the product of all the elements in a finite abelian group $G$ is also equal to the product of all the elements of $G$, i.e., the product of all elements of order at most $2$. In an abelian group, the subset $G$ is a subgroup, so the product has order $2$ and hence squares to the identity element $e$, completing the answer to the OP's question.

But now, on to WTFAG: First of all, if there are no elements of order $2$ then $G = \{e\}$, verifying WTFAG in that case. Second of all, if there is exactly one element $t$ of order $2$, then $G = \{e,t\}$ and the product of all elements in $G$ is equal to $t$.

To complete the proof of WTFAG, one needs to consider the case in which $G$ has more than two elements. In my notes, I do this by first establishing the CLAIM that $G$ (or really any finite group in which every element is self-inverse) is isomorphic to a finite direct product of copies of groups of order $2$, and then using some simple counting arguments.

The CLAIM follows either from the structure theory for finite abelian groups -- which is proved in the same set of notes, but nevertheless the emphasis in the notes is on the fact that in many applications, especially in number theory, the big structure theorem can be avoided -- or from the fact that a group in which each element has order $2$ is necessarily a vector space over the field $\mathbb{F}_2$ of two elements (and then we invoke the structure theorem for finite-dimensional vector spaces). I would actually prefer to have a third, more elementary proof of this fact. Perhaps someone can suggest one?

In addition to Pete Clark's answer, there is also a very neat answer to the question What is the set of all different products of all the elements of a finite group $G$? So $G$ not necessarily abelian. Well, if a 2-Sylow subgroup of $G$ is trivial or non-cyclic, then this set equals the commutator subgroup $G'$. If a 2-Sylow subgroup of $G$ is cyclic, then this set is the coset $xG'$ of the commutator subgroup, with $x$ the unique involution of a 2-Sylow subgroup.

• $@$Nicky: +1! This is a very nice generalization of the abelian case (which, in fact, I had been wondering about). Do you have any references to where these facts are discussed? Jul 22, 2011 at 7:59
• @Pete: have to look it up, author was the Hungarian mathematician J. Dénes. See also J. Dénes and P. Hermann, `On the product of all elements in a finite group', Ann. Discrete Math. 15 (1982) 105-109. The theorem also connects to the theory of Latin Squares and so-called complete maps. Jul 22, 2011 at 9:40

HINT:

An element and its inverse are unique, and each element is either its own inverse or not.