proof of embedding theorem for ordered abelian groups of rank one

proof of embedding theorem for ordered abelian groups of rank one

To prove the

theorem

, we shall use the

characterization of ordered groups of rank one

. Using this result, the theorem to be be proved can be reformulated as follows: (Note that we are now using

additive

notation for groups, so the formulation of the

Archimedean property

appears slightly differently than it did in the entry characterizing the

ordered groups

of rank one.)

Theorem.

Let

G

be an

Abelian

ordered group with order (

http://planetmath.org/OrderGroup

) at least 2.  Then

G

enjoys the Archimedean property

(

∀

x

,

y

∈

G

)

x

<

y

<

0

⇒

(

∃

n

>

0

)

n

y

<

x

iff there is an

order-preserving

isomorphism

from

G

onto some

subgroup

of the

additive group

of

real numbers

.

Proof

One of the two

implications

is rather easy. Since the real numbers enjoy the Archimedean property and it is

obvious

that any subset of a set enjoying the Archimedean property also enjoys this

property

, it follows that every subgroup of the additive group of the real numbers enjoys the Archimedean property. It only remains to show that every group enjoying this property is isomorphic to a subgroup of additive group of the real numbers.

To accomplish this, we shall use the technique of

Dedekind cuts

to map our group into the real numbers. Since the order of our group is at least 2, there exists an

element

x

∈

G

such that

x

≠

0

. By the basic theorem on ordered groups, it follows that either

x

>

0

or

x

<

0

and that, if

x

>

0

then

-

x

<

0

. Thus there exists an element

y

∈

G

such that

-

y

<

0

— if

x

<

0

, set

y

=

x

, else set

y

=

-

x

.

We shall now

construct

a map

μ

:

G

→

ℝ

. Define the sets

A

⁢

(

z

)

and

B

⁢

(

z

)

as follows:

A

⁢

(

z

)

=

{

(

m

,

n

)

∈

ℤ

×

ℕ

∣

m

⁢

y

<

n

⁢

z

}

B

⁢

(

z

)

=

{

(

m

,

n

)

∈

ℤ

×

ℕ

∣

m

⁢

y

>

n

⁢

z

}

By Corollary 2 to the basic theorem on ordered groups, it follows that, for any

strictly

positive integer

k

, we have

(

m

,

n

)

∈

A

⁢

(

z

)

if and only if

(

k

⁢

m

,

k

⁢

n

)

∈

A

⁢

(

z

)

. Likewise,

(

m

,

n

)

∈

B

⁢

(

z

)

if and only if

(

k

⁢

m

,

k

⁢

n

)

∈

B

⁢

(

z

)

. This means that

A

⁢

(

z

)

and

B

⁢

(

z

)

may be regarded as sets of rational numbers, and we can rewrite the defining equations as

A

⁢

(

z

)

=

{

m

/

n

∈

ℚ

∣

m

⁢

y

<

n

⁢

z

}

B

⁢

(

z

)

=

{

m

/

n

∈

ℚ

∣

m

⁢

y

>

n

⁢

z

}

We shall now show that

(

A

⁢

(

z

)

,

B

⁢

(

z

)

)

is a Dedekind cut. This requires us to verify the three

defining properties

of a Dedekind cut. First, we need to check that neither

A

⁢

(

z

)

nor

B

⁢

(

z

)

are empty. We shall only present the proof for

A

⁢

(

z

)

since the proof for

B

⁢

(

z

)

can easily be obtained from it by reversing the

inequality

signs suitably. By the basic theorem, either

-

y

<

z

or

z

=

-

y

or

z

<

-

y

. In the first case,

1

/

1

∈

A

⁢

(

z

)

. In the second case,

2

/

1

∈

A

⁢

(

z

)

. In the third case, the Archimedean property implies that there exists

n

such that

-

1

<

n

⁢

z

<

0

, so

1

/

n

∈

A

⁢

(

z

)

. Second, we need to check that every element of

A

⁢

(

z

)

is less than every element of

B

⁢

(

z

)

. Suppose that

m

/

n

∈

A

⁢

(

z

)

and

m

′

/

n

′

∈

B

⁢

(

z

)

, so that

m

⁢

y

<

n

⁢

z

and

n

′

⁢

z

<

m

′

⁢

y

. Then, by Corollary 2 of the basic theorem,

m

⁢

n

′

⁢

y

<

n

⁢

n

′

⁢

z

and

n

⁢

n

′

⁢

z

<

m

′

⁢

n

⁢

y

. By

conclusion

2 of the basic theorem, this implies that

m

⁢

n

′

⁢

y

<

m

′

⁢

n

⁢

y

. By Corollary 2, this means that

m

⁢

n

′

<

m

′

⁢

n

. Dividing by

n

⁢

n

′

, we conclude that

m

/

n

<

m

′

/

n

′

. Third, we need to show that at most one

rational number

does not belong to either

A

⁢

(

z

)

or to

B

⁢

(

z

)

. Suppose that

m

/

n

does not belong to either

A

⁢

(

z

)

or

B

⁢

(

z

)

. Then it cannot be the case that either

m

⁢

y

<

n

⁢

z

or

n

⁢

z

<

m

⁢

y

.

By the first conclusion of the basic theorem, it follows that

m

⁢

y

=

n

⁢

z

. Now suppose that

m

′

/

n

′

also does not belong to either

A

⁢

(

z

)

or to

B

⁢

(

z

)

. By the same line of reasoning, we must have

m

′

⁢

y

=

n

′

⁢

z

. This would imply that

m

′

⁢

n

⁢

y

=

n

⁢

n

′

⁢

z

. Similarly,

m

⁢

n

′

⁢

y

=

n

⁢

n

′

⁢

z

. Combining these two facts using conclusion 2 of the basic theorem, we would have

m

′

⁢

n

=

m

⁢

n

′

, which implies that

m

/

n

=

m

′

/

n

′

.

Since

(

A

⁢

(

z

)

,

B

⁢

(

z

)

)

is a Dedekind cut, it defines a real number. We shall define

μ

⁢

(

z

)

to be this number.

We will now show that

μ

is a homomrphism. To accomplish this, it suffices to show that

μ

⁢

(

a

-

b

)

=

μ

⁢

(

a

)

-

μ

⁢

(

b

)

for every

a

,

b

∈

G

. Suppose that

m

1

/

n

1

∈

A

⁢

(

a

)

and

m

2

/

n

2

∈

B

⁢

(

b

)

(as before, we assume

n

1

,

n

2

>

0

. Then, by

definition

,

m

1

⁢

y

<

n

1

⁢

a

and

m

2

⁢

y

≥

n

2

⁢

b

.

By Corollary 2 of the basic theorem, we obtain

m

1

⁢

n

2

⁢

y

<

n

1

⁢

n

2

⁢

a

and

-

m

2

⁢

n

1

⁢

y

<

-

n

1

⁢

n

2

⁢

b

.

By conclusion 4 of the basic theorem, we have

(

m

1

⁢

n

2

-

m

2

⁢

n

1

)

⁢

y

<

n

1

⁢

n

2

⁢

(

a

-

b

)

.

Therefore,

m

1

⁢

n

2

-

m

2

⁢

n

1

n

1

⁢

n

2

∈

A

⁢

(

a

-

b

)

. Thus, we have shown that, if

m

1

/

n

1

∈

A

⁢

(

a

)

and

m

2

/

n

2

∈

B

⁢

(

b

)

, then

m

2

/

n

2

-

m

2

/

n

2

∈

A

⁢

(

a

-

b

)

. Likewise, if

m

1

/

n

1

∈

B

⁢

(

a

)

and

m

2

/

n

2

∈

A

⁢

(

b

)

, then

m

2

/

n

2

-

m

2

/

n

2

∈

B

⁢

(

a

-

b

)

. By the definition of

subtraction

for Dedekind cuts, this implies that

μ

⁢

(

a

-

b

)

=

μ

⁢

(

a

)

-

μ

⁢

(

b

)

.

Next, we shall show that

μ

preserves

the order. Since we already know that

μ

is a

homomorphism

, it suffices to show that

μ

⁢

(

z

)

≤

0

when

z

≤

0

. By definition of

μ

⁢

(

z

)

as a Dedekind cut, this is

equivalent

to demanding that, whenever

z

≤

0

, then

m

/

n

≥

0

for all

m

/

n

∈

A

⁢

(

z

)

. This follows readily from the fact that, if

z

<

0

and

m

<

0

and

n

>

0

, then

n

⁢

z

<

0

<

m

⁢

y

, so

n

⁢

z

<

m

⁢

y

and hence

m

/

n

could not possibly belong to

A

⁢

(

z

)

.

To the proof, we must show that

μ

is in fact an isomorphism. This may be accomplished by showing that the kernel of

μ

is

trivial

. Suppose that

μ

⁢

(

z

)

=

0

. Then, by the definition of Dedekind cut,

A

⁢

(

z

)

consists of the negative rational numbers and

B

⁢

(

z

)

consists of the positive rational numbers. By the definitions of

A

and

B

this, in turn, means that

m

⁢

y

<

n

⁢

z

whenever

m

>

0

and

n

>

0

and

m

⁢

y

>

n

⁢

z

whenever

m

<

0

and

n

>

0

. By the basic theorem, either

z

<

0

or

z

=

0

or

z

>

0

. If

z

<

0

then, by the Archimedean property, there must exist an

n

such that

n

⁢

z

<

y

. However, this would contradict the assertion that

m

⁢

y

<

n

⁢

z

whenever

m

>

0

and

n

>

0

, so it is not possible to have

z

<

0

. Likewise, if

z

>

0

, then there must exist an

n

>

0

such that

-

y

<

n

⁢

z

, contradicting the assertion that

m

⁢

y

>

n

⁢

z

whenever

m

<

0

and

n

>

0

. The only remaining possibility is to have

z

=

0

. Therefore, the homomorphism

μ

is, in fact, an isomorphism, so

G

is isomorphic to a subset of the additive group of the real numbers.

Q.E.D.

Title

proof of embedding theorem for ordered abelian groups of rank one

Canonical

name

ProofOfEmbeddingTheoremForOrderedAbelianGroupsOfRankOne

Date of creation

2013-03-22 14:55:33

Last modified on

2013-03-22 14:55:33

Owner

rspuzio (6075)

Last modified by

rspuzio (6075)

Numerical id

22

Author

rspuzio (6075)

Entry type

Proof

Classification

msc 20F60

Classification

msc 06A05

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