# Algebraic structures - help with prove or disprove question

• October 31st 2013, 01:26 AM
Stormey
Algebraic structures - help with prove or disprove question
Hi.
I need to prove or disprove the following:

1. Let $B=\left \{ s+tx|s,t\in \mathbb{Q} \right \}$ and $A=B\setminus \left \{ 0+0x \right \}$. A is a Group.
2. Let $B=\left \{ s+tx|s,t\in \mathbb{Z}_5 \right \}$ and $A=B\setminus \left \{ 0+0x \right \}$. A is a Group.

both with the operation:
$(s+tx)*(u+vx)=(su+2vt)+(sv+tu)x$

My intuition says 1 is true and 2 is false (the unit element in 1 is $(1+0\cdot x)=1$, and with simple algebra I can find the inverse element for a given element $(s+tx)\in A$), but I'm not sure about 2.
Can someone please give me some help with it?

• November 1st 2013, 09:55 AM
johng
Re: Algebraic structures - help with prove or disprove question
Hi Stormey,
Do you know about field extensions? Your question is merely a disguised form of the following:
Let F be a field and $x^2-2$ irreducible over F ( $\sqrt2\notin F$). Then there is a field F' containing F and an element x in F' such that x2 = 2 and every element of F' is uniquely expressible as s + tx for some s and t in F. Of course the non-zero elements of F' form a group under multiplication. This is precisely your group in either 1 or 2.

I really wouldn't want to prove the group axioms from scratch -- associativity would be a mess just from the definition of *. You said you could find the inverse of any element in case F = Q, but you didn't say what it is. Except for notation, this will also be the inverse when F is GF(5). For either case,
$(a+bx)^{-1}=-a(2b^2-a^2)^{-1}+b(2b^2-a^2)^{-1}x$
For a and b not both 0, $2b^2-a^2\ne0$ since x^2-2 has no solution in F.
• November 2nd 2013, 12:56 AM
Stormey
Re: Algebraic structures - help with prove or disprove question
Hi johng. thanks for your help.

Eventually, this is (more or less) what I wrote... (this exercise was due to yesterday).
I remembered something about B looks very much like the field $\mathbb{Q}(\sqrt 2)$, but I couldn't put my finger on why it is relevant.
Hope to get some points for this question...