# The reals (constructed as classes of Cauchy sequences of rationals) form a field

The real num­bers, when con­structed as equiv­alence classes of Cauchy se­quences of ra­tio­nals, form a to­tally or­dered field, with the in­her­ited field struc­ture given by

• $$[a_n] + [b_n] = [a_n+b_n]$$

• $$[a_n] \times [b_n] = [a_n \times b_n]$$

• $$[a_n] \leq [b_n]$$ if and only if ei­ther $$[a_n] = [b_n]$$ or for suffi­ciently large $$n$$, $$a_n \leq b_n$$.

# Proof

Firstly, we need to show that those op­er­a­tions are even well-defined: that is, if we pick two differ­ent rep­re­sen­ta­tives $$(x_n)_{n=1}^{\infty}$$ and $$(y_n)_{n=1}^{\infty}$$ of the same equiv­alence class $$[x_n] = [y_n]$$, we don’t some­how get differ­ent an­swers.

## Well-defined­ness of $$+$$

We wish to show that $$[x_n]+[a_n] = [y_n] + [b_n]$$ when­ever $$[x_n] = [y_n]$$ and $$[a_n] = [b_n]$$; this is an ex­er­cise.

Since $$[x_n] = [y_n]$$, it must be the case that both $$(x_n)$$ and $$(y_n)$$ are Cauchy se­quences such that $$x_n - y_n \to 0$$ as $$n \to \infty$$. Similarly, $$a_n - b_n \to 0$$ as $$n \to \infty$$.

We re­quire $$[x_n+a_n] = [y_n+b_n]$$; that is, we re­quire $$x_n+a_n - y_n-b_n \to 0$$ as $$n \to \infty$$.

But this is true: if we fix ra­tio­nal $$\epsilon > 0$$, we can find $$N_1$$ such that for all $$n > N_1$$, we have $$|x_n - y_n| < \frac{\epsilon}{2}$$; and we can find $$N_2$$ such that for all $$n > N_2$$, we have $$|a_n - b_n| < \frac{\epsilon}{2}$$. Let­ting $$N$$ be the max­i­mum of the two $$N_1, N_2$$, we have that for all $$n > N$$, $$|x_n + a_n - y_n - b_n| \leq |x_n - y_n| + |a_n - b_n|$$ by the tri­an­gle in­equal­ity, and hence $$\leq \epsilon$$. <div><div>

## Well-defined­ness of $$\times$$

We wish to show that $$[x_n] \times [a_n] = [y_n] \times [b_n]$$ when­ever $$[x_n] = [y_n]$$ and $$[a_n] = [b_n]$$; this is also an ex­er­cise.

We re­quire $$[x_n a_n] = [y_n b_n]$$; that is, $$x_n a_n - y_n b_n \to 0$$ as $$n \to \infty$$.

## Ring structure

The mul­ti­plica­tive iden­tity is $$$$ (for­mally, the equiv­alence class of the se­quence $$(1,1, \dots)$$). In­deed, $$[a_n] \times  = [a_n \times 1] = [a_n]$$.

$$\times$$ is closed, be­cause the product of two Cauchy se­quences is a Cauchy se­quence (ex­er­cise).

If $$(a_n)$$ and $$(b_n)$$ are Cauchy se­quences, then let $$\epsilon > 0$$. We wish to show that there is $$N$$ such that for all $$n, m > N$$, we have $$|a_n b_n - a_m b_m| < \epsilon$$.

But $$|a_n b_n - a_m b_m| = |a_n (b_n - b_m) + b_m (a_n - a_m)| \leq |b_m| |a_n - a_m| + |a_n| |b_n - b_m|$$$by the tri­an­gle in­equal­ity. Cauchy se­quences are bounded, so there is $$B$$ such that $$|a_n|$$ and $$|b_m|$$ are both less than $$B$$ for all $$n$$ and $$m$$. So pick­ing $$N$$ so that $$|a_n - a_m| < \frac{\epsilon}{2B}$$ and $$|b_n - b_m| < \frac{\epsilon}{2B}$$ for all $$n, m > N$$, the re­sult fol­lows. <div><div> $$\times$$ is clearly com­mu­ta­tive: $$[a_n] \times [b_n] = [a_n \times b_n] = [b_n \times a_n] = [b_n] \times [a_n]$$. $$\times$$ is as­so­ci­a­tive: $$[a_n] \times ([b_n] \times [c_n]) = [a_n] \times [b_n \times c_n] = [a_n \times b_n \times c_n] = [a_n \times b_n] \times [c_n] = ([a_n] \times [b_n]) \times [c_n]$$$

$$\times$$ dis­tributes over $$+$$: we need to show that $$[x_n] \times ([a_n]+[b_n]) = [x_n] \times [a_n] + [x_n] \times [b_n]$$. But this is true: $$[x_n] \times ([a_n]+[b_n]) = [x_n] \times [a_n+b_n] = [x_n \times (a_n+b_n)] = [x_n \times a_n + x_n \times b_n] = [x_n \times a_n] + [x_n \times b_n] = [x_n] \times [a_n] + [x_n] \times [b_n]$$\$

## Field structure

To get from a ring to a field, it is nec­es­sary and suffi­cient to find a mul­ti­plica­tive in­verse for any $$[a_n]$$ not equal to $$$$.

Since $$[a_n] \not = 0$$, there is some $$N$$ such that for all $$n > N$$, $$a_n \not = 0$$. Then defin­ing the se­quence $$b_i = 1$$ for $$i \leq N$$, and $$b_i = \frac{1}{a_i}$$ for $$i > N$$, we ob­tain a se­quence which in­duces an el­e­ment $$[b_n]$$ of $$\mathbb{R}$$; and it is easy to check that $$[a_n] [b_n] = $$.

$$[a_n] [b_n] = [a_n b_n]$$; but the se­quence $$(a_n b_n)$$ is $$1$$ for all $$n > N$$, and so it lies in the same equiv­alence class as the se­quence $$(1, 1, \dots)$$.

## Order­ing on the field

We need to show that:

• if $$[a_n] \leq [b_n]$$, then for ev­ery $$[c_n]$$ we have $$[a_n] + [c_n] \leq [b_n] + [c_n]$$;

• if $$ \leq [a_n]$$ and $$ \leq [b_n]$$, then $$ \leq [a_n] \times[b_n]$$.

We may as­sume that the in­equal­ities are strict, be­cause if equal­ity holds in the as­sump­tion then ev­ery­thing is ob­vi­ous.

If $$[a_n] = [b_n]$$, then for ev­ery $$[c_n]$$ we have $$[a_n] + [c_n] = [b_n] + [c_n]$$ by well-defined­ness of ad­di­tion. There­fore $$[a_n] + [c_n] \leq [b_n] + [c_n]$$.

If $$ = [a_n]$$ and $$ \leq [b_n]$$, then $$ =  \times [b_n] = [a_n] \times [b_n]$$, so it is cer­tainly true that $$ \leq [a_n] \times [b_n]$$. <div><div>

For the former: sup­pose $$[a_n] < [b_n]$$, and let $$[c_n]$$ be an ar­bi­trary equiv­alence class. Then $$[a_n] + [c_n] = [a_n+c_n]$$; $$[b_n] + [c_n] = [b_n+c_n]$$; but we have $$a_n + c_n \leq b_n + c_n$$ for all suffi­ciently large $$n$$, be­cause $$a_n \leq b_n$$ for suffi­ciently large $$n$$. There­fore $$[a_n] + [c_n] \leq [b_n] + [c_n]$$, as re­quired.

For the lat­ter: sup­pose $$ < [a_n]$$ and $$ < [b_n]$$. Then for suffi­ciently large $$n$$, we have both $$a_n$$ and $$b_n$$ are pos­i­tive; so for suffi­ciently large $$n$$, we have $$a_n b_n \geq 0$$. But that is just say­ing that $$ \leq [a_n] \times [b_n]$$, as re­quired.

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