# The set of all permutations of a A with composition operation is a group:How to show?

• Dec 5th 2012, 07:47 AM
x3bnm
The set of all permutations of a A with composition operation is a group:How to show?
Let $\displaystyle G$ be the subset of $\displaystyle S_4$ consisting of the permutations:

$\displaystyle \varepsilon = \begin{pmatrix} 1 & 2 & 3 & 4\\ 1 & 2 & 3 & 4 \end{pmatrix}$

$\displaystyle f = \begin{pmatrix} 1 & 2 & 3 & 4\\ 2 & 1 & 4 & 3 \end{pmatrix}$

$\displaystyle g = \begin{pmatrix} 1 & 2 & 3 & 4\\ 3 & 4 & 1 & 2 \end{pmatrix}$

$\displaystyle h = \begin{pmatrix} 1 & 2 & 3 & 4\\ 4 & 3 & 2 & 1 \end{pmatrix}$

Now show that $\displaystyle G$ is a group of permutations.

Now the problem is how do I prove this? Because I know that in order to be a group of permutations I've to show the following $\displaystyle 4$ properties:

a) The operation $\displaystyle \circ$ of composition of functions qualifies as an operation on the set of all the permutations of $\displaystyle A$

b) This operation is associative.

c) There is a permutation $\displaystyle \varepsilon$ such that $\displaystyle \varepsilon \circ f = f$ and $\displaystyle f \circ \varepsilon = f$ for any permutation $\displaystyle f$ of $\displaystyle A$.

d) Finally for every permutation $\displaystyle f$ of $\displaystyle A$ there is another permutation $\displaystyle f^{-1}$ of $\displaystyle A$ such that $\displaystyle f \circ f^{-1} = \varepsilon$
and $\displaystyle f^{-1} \circ f = \varepsilon$

Do I exhaust all the possible values and show that the above four properties hold true for all combinations?

Is there any easy way to prove that the set of all the above permutations is a group?
• Dec 5th 2012, 08:28 AM
Plato
Re: The set of all permutations of a A with composition operation is a group:How to s
Quote:

Originally Posted by x3bnm
Let $\displaystyle G$ be the subset of $\displaystyle S_4$ consisting of the permutations:
$\displaystyle \varepsilon = \begin{pmatrix} 1 & 2 & 3 & 4\\ 1 & 2 & 3 & 4 \end{pmatrix}$
$\displaystyle f = \begin{pmatrix} 1 & 2 & 3 & 4\\ 2 & 1 & 4 & 3 \end{pmatrix}$
$\displaystyle g = \begin{pmatrix} 1 & 2 & 3 & 4\\ 3 & 4 & 1 & 2 \end{pmatrix}$
$\displaystyle h = \begin{pmatrix} 1 & 2 & 3 & 4\\ 4 & 3 & 2 & 1 \end{pmatrix}$
Now show that $\displaystyle G$ is a group of permutations.
Is there any easy way to prove that the set of all the above permutations is a group?

Well it is it is easy to see that each of those is its own inverse.
Moreover, the set is closed under the operation.
• Dec 5th 2012, 09:05 AM
x3bnm
Re: The set of all permutations of a A with composition operation is a group:How to s
Quote:

Originally Posted by Plato
Well it is it is easy to see that each of those is its own inverse.
Moreover, the set is closed under the operation.

Thanks Plato for help.
• Dec 5th 2012, 09:25 AM
Deveno
Re: The set of all permutations of a A with composition operation is a group:How to s
b) is obvious: composition of functions is associative:

if h(x) = y, and g(y) = z and f(z) = w, then:

(fo(goh))(x) = f((goh)(x)) = f(g(h(x))) = f(g(y)) = f(z) = w

((fog)oh)(x) = (fog)(h(x)) = (fog)(y) = f(g(y)) = f(z) = w.

as Plato pointed out, every element (function) is its own inverse, so we have (d).

(c) is redundant: closure (a) & inverses (d) together imply (c). you can safely avoid proving this (it will be true: if a*b is in S whenever a,b are, and a-1 is in S when a is, then if a is in S, then so is a-1, so taking b = a-1, we have:

a*b = a*a-1 = e is in S).

there is a trick you can use to show closure:

let a = (1 2)(3 4) (this is f in your original post...by (1 2)(3 4) i mean f(1) = 2, f(2) = 1, f(3) = 4, and f(4) = 3).
let b = (1 3)(2 4) (this is g in your original post).

show your last element is a*b.

this means that G = {e,a,b,a*b}

show that a*b = b*a. this implies that G is abelian (why? hint: a and b are generators...why?). this cuts down the number of products you have to verify closure for down from 16 to 10. 4 of those you will already have verified by proving you have inverses (the ones of the form g*g). that leaves just 6 to check. if you happen to notice that ε is the identity function on {1,2,3,4}, this makes 3 of the remaining products to check for closure trivial. that leaves just three products you have to work for:

a*b <---but you'll already have calculated this verifying a*b = b*a
a*(a*b) <---asociativity and inverses make this trivial
b*(a*b) <---use associativity and the fact that b*a = a*b, and what you learned about inverses.

extra credit: show that G is isomorphic to the klein 4-group V (for vierergruppe, german for "four-group"), and also to the direct product: (Z2xZ2,+) (where + means addition mod 2, the elements are:

{(0,0),(1,0),(0,1),(1,1)} you can think of this as "a pair of unlinked switches").
• Dec 5th 2012, 09:38 AM
Plato
Re: The set of all permutations of a A with composition operation is a group:How to s
Quote:

Originally Posted by x3bnm
Let $\displaystyle G$ be the subset of $\displaystyle S_4$ consisting of the permutations: Is there any easy way to prove that the set of all the above permutations is a group?

I assumed that you know that $\displaystyle S_4$ is a group.
There is a standard theorm: If $\displaystyle H$ is a subset of a group $\displaystyle G$ then $\displaystyle H$ is a subgroup of $\displaystyle G$ if and only if $\displaystyle \{a,b\}\subset H$ implies that $\displaystyle ab^{-1}\in H$.