# Math Help - sum, difference and product of two null-sequence (need some help)

1. ## [SOLVED] sum, difference and product of two null-sequence (need some help)

It's like this.
I didn't have any problems to prove some of definitions of sequences like :

$lim_{n->+\infty}{(ax_n+by_n)}=ax+by$

Let
$lim_{n->+\infty}{x_n}=x$
and
$lim_{n->+\infty}{y_n}=y$
for any $\xi>0$. Starting from some index $n_1$ every member of sequence $(x_n)$ are in $\xi$-region of point x. Same, from some index $n_2$ every member of sequence $(y_n)$ are in $\xi$-region of point y.

If we put $n_0=max {\{n_1 ,n_2 \}}$ then for every $n >= n_0$ these inequality are true :

$|x_n-x|<\xi$ and $|y_n-y|<\xi$

so from inequality of triangle for $n >= n_0$ we have:

$|ax_n+by_n-(ax+by) |=|ax_n-ax+by_n-by|\le |a||x_n-x|+|b||y_n-y| \le (|a|+|b| )\xi$

Because $(|a|+|b|)$ is fix real number and $\xi$ any small number so it's $(|a|+|b| )\xi$ any small number so there we have that is:

$lim_{n->+\infty}{(ax_n+by_n)}=ax+by$

true.

This and some another I got it OK

But I can't, or don't see how to prove (or show) that sum of two null-sequences are again null-sequence (and difference of two sequences) becose it's to obviously. And for the product of two null-sequences is null-sequence and how we can relax conditions for the product of two null-sequences?

any help will be most appreciated
Thanks very much

2. Originally Posted by yeKciM
It's like this.
I didn't have any problems to prove some of definitions of sequences like :

$lim_{n->+\infty}{(ax_n+by_n)}=ax+by$

Let
$lim_{n->+\infty}{x_n}=x$
and
$lim_{n->+\infty}{y_n}=y$
for any $\xi>0$. Starting from some index $n_1$ every member of sequence $(x_n)$ are in $\xi$-region of point x. Same, from some index $n_2$ every member of sequence $(y_n)$ are in $\xi$-region of point y.

If we put $n_0=max {\{n_1 ,n_2 \}}$ then for every $n >= n_0$ these inequality are true :

$|x_n-x|<\xi$ and $|y_n-y|<\xi$

so from inequality of triangle for $n >= n_0$ we have:

$|ax_n+by_n-(ax+by) |=|ax_n-ax+by_n-by|\le |a||x_n-x|+|b||y_n-y| \le (|a|+|b| )\xi$

Because $(|a|+|b|)$ is fix real number and $\xi$ any small number so it's $(|a|+|b| )\xi$ any small number so there we have that is:

$lim_{n->+\infty}{(ax_n+by_n)}=ax+by$

true.

This and some another I got it OK

But I can't, or don't see how to prove (or show) that sum of two null-sequences are again null-sequence (and difference of two sequences) becose it's to obviously.
Well, that's funny: you just wrote that you have no trouble believing (even proving) that

$\lim_{n\to\infty}(ax_n+by_n)=ax+by$

is true, provided $x_n\to x, y_n\to y$ for $n\to \infty$.

Now, if $(x_n), (y_n)$ are null-sequences, then what you know is that any linear combination $z_n := ax_n+by_n$ is, again a null sequence, because in that case you have $z_n\to z=a\cdot 0+b\cdot 0=0$.

If you know that from $x_n\to x$ and $y_n\to y$ it follows that $x_n y_n\to x y$, then it follows again, that the product of two null-sequences (which jus means that x=0 and y=0), is again a null-sequence $z_n := x_n\cdot y_n\to 0\cdot 0=0$.

And for the product of two null-sequences is null-sequence and how we can relax conditions for the product of two null-sequences?
As to relaxing the restrictions on the product of null-sequences: if you multiply a null-sequence $x_n\to 0$ with a bounded sequence $y_n$ (which means that there exists a constant M such that for all n $|y_n|\leq M$), you get that the product $z_n := x_n \cdot y_n$ is also a null- sequence.

3. Thanks very very much !!!

It seems to me that's not enough... that's why i didn't post any my attempts of proofing thath one

It just seems to simple i don't like obviously proofs

THANKS!!!!