I am given $((0,1),*)$ Where $x,y\in (0,1)$ and $*$ is defined as $x*y=\frac{xy}{1-x-y+2xy}$ How should I go about finding the inverse and identity elements?
Prove abelian group
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1Once you prove that this operation defines a group structure, then it'll be clearly abelian because the expression is symmetric in $x$ and $y$. – 2012-03-20
3 Answers
Hint:
Let $e$ be a identity in $((0,1), \ast)$ and $x^{-1}$ denote a inverse of $x$.
$x \ast e=x$ for all $x$.
$x \ast x^{-1}=e$
From your answer, prove that they are the identity and the inverse respectively.
That the answer has been spelt out, I'll leave you with another exercise (Sorry!).
Try this exercise:
(Dec. 2011, Math. Reflections) On the set $M~=~\mathbb R-\{3\}$, the following binary law is defined: $ x \ast y = 3(xy-3x-3y)+m$ where $m \in \Bbb R$. Find the values of $m$ such that $(M, \ast)$ forms a group.
(Proposed to Math. Reflections by Bogdan Enescu, ``B. P. Hasdeu" National College, Buzau, Romania)
Ping me here in case you want hints, and if you want an answer google, Math. Reflections and I have given you the reference for the problem here. I had written a solution to Math. Reflections, which if you're particular, I'll add it here later.
If the problem is to verify that $G = ((0, 1), \ast)$ is a group, in fact what we have to show the first is whether the associativity holds or not. Let
$ f(z) = \frac{1-z}{z} $
with the inverse
$ f^{-1}(w) = \frac{1}{1+w}.$
Then
$\frac{xy}{1-x-y+2xy} = \frac{xy}{(1-x)(1-y) + xy} = \frac{1}{\left(\frac{1-x}{x} \right)\left( \frac{1-y}{y} \right) + 1} = f^{-1}(f(x)f(y)),$
thus we obtain
$ \begin{align*} x \ast (y \ast z) & = x \ast f^{-1}(f(y)f(z)) = f^{-1}(f(x)f(y)f(z)) \\ (x \ast y) \ast z & = f^{-1}(f(x)f(y)) \ast z = f^{-1}(f(x)f(y)f(z)), \end{align*}$
so that they coincide. Furthermore, this shows that $f$ is an isomorphism from $G$ to the group $(\mathbb{R}^{+}, \cdot)$ of positive real numbers equipped with usual multiplication. Thus both identity and the inverse can be trace back from this isomorphism as follows:
$ \begin{align*} e &= f^{-1}(1) = \frac{1}{2}. \\ x^{-1} &= f^{-1}\left( \tfrac{1}{f(x)} \right) = 1 - x. \end{align*}$
In fact, most of artificial operations $\ast$ in the problems are given in this way. That is, they are a disguise of some familiar operations \ast' driven by some bijection $f$, so that the operation takes the form
x \ast y = f^{-1}(f(x) \ast' f(y)).
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0I particularly enjoyed your final observation about how operations like this one are manufactured. – 2012-03-21
Well, to find the identity, you need to find an element $y$ such that for every $x$, $x*y = \frac{xy}{1-x-y+2xy} = x.$ This leads to $xy = x - x^2 - xy + 2x^2y$ or $x^2-x = (2x^2-2x)y$ hence to $y = \frac{x^2-x}{2x^2-2x} = \frac{1}{2}.$ Now verify that $\frac{1}{2}$ is actually the identity of this operation.
Once you know the identity, you can try to find inverses. Given $x\in (0,1)$, you are looking for a $z$ such that $x*z = \frac{1}{2}$ (assuming the above was correct). Solve for $z$ in terms of $x$ and verify that it lies in your set.
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1@Mike: Of course, you have to verify $*$ is an associative operation on $(0,1)$ (if $x,y\in (0,1)$, then $x*y\in (0,1)$; and if $x,y,z\in (0,1)$, then $(x*y)*z = x*(y*z)$) if you want to prove you have a group. – 2012-03-20