# The difference between the limits of two Cauchy Sequences

1. Jul 5, 2013

### ANphysics

Lets say that we have two Cauchy sequences {fi} and {gi} such that the sequence {fi} converges to a limit F and the sequence {gi} converges to a limit G. Then it can easily be shown that the sequence defined by
{ d(fi, gi) } is also Cauchy.

But is it true that this sequence, { d(fi, gi) }, converges to the limit d(F,G) ?

Last edited: Jul 5, 2013
2. Jul 5, 2013

### HallsofIvy

Staff Emeritus
Yes, it is. If $\{f_i\}$ converges to F then, given any $\epsilon> 0$, there exist N such that if n> N then $|f_n- F|< \epsilon/2$. If $\{g_i\}$ converges to G then, given any $\epsilon> 0$, there exist M such that if n> M, then $|g_n- G|< \epsilon/2$.

Given any $\epsilon> 0$ if n> the larger of M and N, we have $|(f_i- g_i)- (F- G)|\le |f_i- F|+ |g_i- G|< \epsilon$.

3. Jul 5, 2013

### ANphysics

But aren't you assuming here that distance is defined to be the absolute value of the difference between two elements. Is this true in general for any metric space?

For example, we can consider the vector space of all functions defined on [0,1] and define an inner product to be < f(x), g(x)> = ∫f(x)g(x)dx and a distance function to be
d(f(x),g(x))= (∫(f-g)(f-g)dx)1/2. So in general when you have a metric space, but you don't necessarily know what that distance function is, can this statement be derived from the definition of metric space?

Last edited: Jul 5, 2013
4. Jul 5, 2013

### Mandelbroth

Let's consider a discrete metric, which I shall denote r(x,y), and the case that $F=G$, but $f_i\neq g_i$. Then, the limit of $\{r(f_i,g_i)\}$ converges to 1, but $r(F,G)=0$.

As a fun little exercise, consider what happens when the metric is continuous. :tongue:

5. Jul 6, 2013

### jbunniii

But if the metric is discrete, then $f_i \rightarrow F$ implies that $f_i = F$ for sufficiently large $i$, and similarly $g_i = G$ for sufficiently large $i$. Therefore we must eventually have $f_i = F = G = g_i$ if $i$ is large enough.

6. Jul 6, 2013

### jbunniii

The question is whether the distance metric $d$ is a continuous function. This is true in any metric space. To see this, let $\epsilon > 0$, and note that $f_i \rightarrow F$ and $g_i \rightarrow G$ imply that for sufficiently large $i$, we have $d(f_i, F) < \epsilon/2$ and $d(g_i, G) < \epsilon / 2$.

Now we just play around with the triangle inequality to get what we need. First note the triangle inequality for real numbers gives us
$$|d(f_i, g_i) - d(F, G)| \leq |d(f_i, g_i) - d(f_i, G)| + |d(f_i, G) - d(F,G)|$$
But the triangle inequality for the metric $d$ gives us
$$|d(f_i, g_i) - d(f_i, G)| \leq d(g_i, G)$$
and
$$|d(f_i, G) - d(F,G)| \leq d(f_i, F)$$
so the first inequality becomes
$$|d(f_i, g_i) - d(F, G)| \leq d(f_i, F) + d(g_i, G)$$
which is bounded above by $\epsilon/2 + \epsilon/2 = \epsilon$ for sufficiently large $i$.

7. Jul 6, 2013

### jbunniii

I guess I'm not seeing your point. The counterexample you offered does not work, for the reason I noted, regardless of what underlying set is given the discrete metric. Any convergent sequence under the discrete metric must be eventually constant. Therefore two convergent sequences with the same limit must be eventually equal to each other.

8. Jul 6, 2013

### WannabeNewton

Last edited: Jul 6, 2013
9. Jul 6, 2013

### Mandelbroth

My apologies. I forgot that Cauchy sequences are defined by a metric when not working with real numbers. I was talking nonsense, so there was no point to see.

10. Jul 6, 2013

### WannabeNewton

My significant other jbun says there is no need to apologize.

11. Jul 6, 2013

### jbunniii

If I had a dollar for every time I've spoken nonsense, I could afford to buy WBN a pony.

12. Jul 6, 2013

### WannabeNewton

I want a pony :[

13. Jul 6, 2013

### jbunniii

14. Jul 6, 2013

### WannabeNewton

Do the girls come with the shirts as a package deal? xP