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Let $(G, \cdot)$ be a group. Define $(G, *)$ as a group with the same underlying set and an operation $$a * b := b \cdot a.$$ What do you call such a group? What is the usual notation for it? I tried searching for 'dual group' and 'opposite group' with no results. It seems that this group is all you need to dispose of having to explicitly define right action, but there is no mention of such a group in wiki's 'Group action'. Am I mistaken and it's not actually a group?

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    Of course it's a group. $(a * b) * c = c \cdot (b \cdot a) = (c \cdot b) \cdot a = a* (b * c)$. Inverse and identity are obvious.2011-04-02
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    I figured that out, but you can never be too certain :)2011-04-02
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    I'm getting this as a first result by googling for "opposite group": http://planetmath.org/encyclopedia/OppositeGroup.html All you want to know seems to be answered there and $G^{\operatorname{op}}$ is the usual notation. A detailed verification that $G^{\operatorname{op}}$ is a group can be found in the second google hit: http://www.proofwiki.org/wiki/Opposite_Group2011-04-02
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    OK, I should add it to wiki then :) I'll accept it as an answer if you post it as such.2011-04-02
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    A related question on MathOverflow: http://mathoverflow.net/questions/2566/silly-question-about-opposite-groups2011-04-02
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    OK, I copypasted from planetmath to Wikipedia (I hope CC-BY-SA and CC-SA are compatible <_<) I also added a bit on my own, please see if it's all correct :)2011-04-02
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    Alexei: thank you. As I don't have a wikipedist's account and don't want to open one for this trivial remark, I'm posting this here: maybe you could add the line "Also, a left action of $G$ on $X$ is the same thing as a right action of the opposite group $G^{\operatorname{op}}$ on $X$ and vice versa." (with the proper linking, of course) at the very end of the definition section on http://en.wikipedia.org/wiki/Group_action2011-04-02
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    OK, I'll do that. Also, you don't need an account to edit Wikipedia, just press 'Edit' :)2011-04-02
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    Thanks, good to know that... I guess I was always scared off by this big yellow thingy popping up and complaining that I'm not logged in. I never took the pains of reading (only a tiny little bit) further :p2011-04-02
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    @t.b. the planet math link is dead.2017-01-13

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I'm getting this as a first result by googling for opposite group: PlanetMath: opposite group.

All you want to know seems to be answered there and $G^{\operatorname{op}}$ is the usual notation.

A detailed verification that $G^{\operatorname{op}}$ is a group can be found by following the second google hit: Opposite Group - ProofWiki

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    Indeed, this is just a special case of taking the opposite of a category (http://ncatlab.org/nlab/show/opposite+category) in the case where the category (is small and) has one object and all morphisms invertible. Note that in the special case of groups every group is canonically isomorphic to its opposite group via the map $g \mapsto g^{-1}$. This is the map that allows you to freely pass between left and right actions.2011-04-02
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A group is a groupoid (meaning a category all of whose arrows are invertible) with one object. If $\star$ denotes the object, group elements correspond to arrows $\star\to \star$, and multiplication $gh$ corresponds to composition $\star\xrightarrow{h} \star\xrightarrow{g} \star$. The opposite category of this category is the opposite group of this group: domain and codomain are reversed, so the old $gh$ is the new $hg$.

\Edit: Qiaochu Yuan was faster than me.

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    +1: nonetheless :) This is also pointed out under the PlanetMath link I gave in my "answer".2011-04-02