Well there is these rules:
(lim n->oo an)*(lim n->oo bn) = lim n->oo (an*bn)
(lim n->oo an)+(lim n->oo bn) = lim n->oo (an+bn)
Thats why Cauchy sequences are clause concerning
multiplication and additiona. Then it is only a small
step to see that Cauchy sequences limits are real numbers,
real numbers admiting multiplication and addition.
For a proof that multiplication {an*bn} and addition
{an+bn} are also Cauchy if the original {an} and {bn}
where Cauchy, just observer:
Proof:
By definition, since we assume {an} Cauchy, for every e,
there is an N, such:
forall n,m >= N(e) |an-am| =< e
And the same for {an}, since we assume it Cauchy as
well, for every d there is an M, such that:
forall n,m >= M(d) |bn-bm| =< d
Now assume we have a further epsilon g, lets see whether
we can admister an index K and J, such that:
forall n,m >= K |(an*bn)-(am*bm)| =< g
forall n,m >= J |(an+bn)-(am+bm)| =< g
Lets first start with addition, this is simpler,
we have |(an+bn)-(am+bm)| = |an-am+bn-bm|, by
the triangle inequality, we have:
|an-am+bn-bm| =< |an-am|+|bn-bm|
Now use J=max(N(g/2),M(g/2)), we clearly have:
forall n,m >=J |an-am| =< g/2
forall n,m >=J |bn-bm| =< g/2
Amd therefore:
forall n,m >=J |an-am|+|bn-bm| =< g
And hence as required:
forall n,m >=J |(an+bn)-(am+bm)| =< g
Now lets try multipiclation: We have |(an*bn)-(am*bm)|=
|an*(bn-bm)+bm*(an-am)|, by the triangle inequality,
we have:
|(an*bn)-(am*bm)| =< |an*(bn-bm)|+|bm*(an-am)|
= |an|*|bn - bm| + |bm|*|an-am|
Now we use the property that Cauchy sequence are bounded,
they never tend to +oo or -oo, there fore we have
A with |an|=<A and B with |bm|=<B irrespective of the
index n or m. And hence:
=< A*|bn - bm| + B*|an-am|
Now use K=max(N(g/(2*B)),M(g/(2*A))), we clearly have:
forall n,m >=K |an-am| =< g/(2*B)
forall n,m >=K |bn-bm| =< g/(2*A)
Or respectively:
forall n,m >=K B*|an-am| =< g/2
forall n,m >=K A*|bn-bm| =< g/2
Amd therefore:
forall n,m >=K A*|bn-bm|+B*|an-am| =< g
And hence as required:
forall n,m >=K |(an*bn)-(am*bm)| =< g
For the multiplication proof see also here:
Proving that product of two Cauchy sequences is Cauchy
https://math.stackexchange.com/questions/376324/proving-that-product-of-two-cauchy-sequences-is-cauchy
Cauchy Sequence: Multiplication Property
https://math.stackexchange.com/questions/309867/cauchy-sequence-multiplication-property