Proving Q[22/3] is a Subring of R

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In summary, We are trying to prove that the set Q[22/3]={a+b22/3+c42/3} is a subring of the real numbers, denoted by R. The set is still an Abelian group under addition and has additive identity 0. It also has additive inverses and is closed under multiplication. However, it is necessary to show that the operations of the larger ring (C, the complex numbers) on this subset give the same values as those operations on the subset. This is not enough to prove that it is a subring, as there may be additional requirements depending on the definition used. It is important to clarify whether the definition of "subring" includes the multiplicative identity,
  • #1
tom.young84
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Prove that Q[22/3]={a+b22/3+c42/3} is a subring of the R. Here Q denotes rationals and R denotes reals (I'm not sure how to do the boldface).

In my algebra class we haven't gotten to the point in which we can just show that it is a subring through subtraction and multiplication.

This is still a abelian group under addition:
- (a+b22/3+c42/3)+(c+d22/3+f42/3)=(a+c)+(b22/3+d22/3)+(c42/3+f42/3); closure
- It has additive identity 0, 0 is in C
- Additionally it has additive inverses

For multiplication:
- one can multiply two elements and still get an element in the ring

Is this sufficient?
 
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  • #2
You forgot the multiplicative identity, and the rest of the ring axioms.


But there is a problem: this would merely show it to be a subset and a ring, but it's not enough to be a subring! You have to remark that 0,1,+,-,* for the larger ring have to, when applied to the smaller ring, give the same values as those operations on the smaller ring.


There may be another problem -- what precisely do you mean by {a+b22/3+c42/3}?
 
  • #3
The larger ring is C (the complex number), we just have to show that this subset is in fact a subring. I thought a subring needed to suffice 4 axioms: closure under addition, additive inverse, additive identity, and closure under multiplication

Additionally, I forgot to answer this. It's the cube root, I misinterpreted it.

a+b[tex]^3\sqrt[/tex]2+c[tex]^3\sqrt[/tex]4
 
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  • #4
I was more wondering if a+b22/3+c42/3 was supposed to be a formal linear combination, or if it was supposed to be the real number named by that linear combination.


In the general context I'm used to, the phrase "A is a subXXX of B" is defined to mean "A is an XXX, B is an XXX, the underlying set of A is a subset of the underlying set of B, and the inclusion function on those sets defines a homomorphism of XXX's". So I was speaking to that definition.


But if you are using a different definition -- and there are lots of equivalent ways to define things -- then what you need to prove will need to be geared for that definition instead.

As an FYI, I feel like making something explicit: something implicit in your definition is that it's not saying "A is a subring", but instead "A, together with the 0, 1, +, -, * formed by taking the operations of B and restricting them to A, is a subring of B"
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All that said... are you really sure your definition of "subring" doesn't say anything about 1?

I've been assuming all along that "ring"s have to include the multiplicative identity -- and thus to show something is a subring, it has to include it.

However, there is another convention that defines "ring" without any reference to multiplicative identity. In that case, it would have nothing to do with whether something is a subring.
 

FAQ: Proving Q[22/3] is a Subring of R

What is a subring?

A subring is a subset of a ring that is closed under addition, subtraction, and multiplication. It also contains the additive and multiplicative identities of the original ring, and is itself a ring under the same operations and with the same identities.

How do you prove that Q[22/3] is a subring of R?

To prove that Q[22/3] is a subring of R, we need to show that it meets all the criteria of a subring. This includes showing that it is a subset of R, that it is closed under addition, subtraction, and multiplication, and that it contains the identities 0 and 1. We also need to show that it is a ring under the same operations and identities as R.

Why is it important to prove that Q[22/3] is a subring of R?

Proving that Q[22/3] is a subring of R is important because it allows us to understand the relationship between these two mathematical structures. It also allows us to use properties and theorems from the larger ring R to solve problems involving the subring Q[22/3]. Additionally, it helps us to see how subrings can be created by taking a subset of elements from a larger ring.

What are some common mistakes when trying to prove Q[22/3] is a subring of R?

Some common mistakes when trying to prove Q[22/3] is a subring of R include not showing that it meets all the criteria of a subring (subset, closure, identities), assuming that Q[22/3] is a field instead of a ring, and using incorrect or incomplete definitions of subring and ring operations.

Can Q[22/3] be a subring of any other ring besides R?

No, Q[22/3] can only be a subring of R. This is because any other ring would have different operations and identities, and Q[22/3] would not satisfy the criteria for being a subring of that particular ring.

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