# Cauchy sequences

1. Nov 5, 2008

### garyljc

By definition, a sequence a(n) has the Cauchy sequence if for eery E>0 ,there exist a natural number N such that Abs(a(n) - a(m) ) < E for all n, m > N

Could anyone tell me what is a(m) ? is it a subsequence of a(n) , or could it be any other non related sequence ?

2. Nov 5, 2008

### Office_Shredder

Staff Emeritus
a(m) is the same sequence as a(n)

3. Nov 5, 2008

### lurflurf

a(m) and a(n) are not sequences they are elements of a sequence

Pehaps the difficulty will be eased by restating the definition differently

a sequence is Cauchy if for any E>0 there exist a natural number N such that the difference between any two terms beyond N cannot exceed N

or

a sequence is Cauchy if for any E>0 there exist a natural number N such that Abs(a(N+n) - a(N+m) ) < E for all n,m that are natural numbers

4. Nov 5, 2008

### HallsofIvy

Staff Emeritus
Neither a_m nor a_n in that is a sequence. They are, rather, any two numbers from the original sequence {a_i}, with, of course, m and n larger than N.

5. Nov 5, 2008

### garyljc

OK thanks

One more question
what's the difference between Lim sup a(n) and sup A(n)
does the limit tells me something else ?

6. Nov 5, 2008

### mathman

It is easier to explain by example. Consider the sequence 1, 1/2, 1/3, 1/4,...

The sup is 1, while the lim sup is 0.

7. Nov 10, 2008

what basically is the change that lim produced in sup
why it changes sup=1
to lim sup=0
and do this thing hold in all cases that lim sup is not the part of the sequence

8. Nov 11, 2008

### Pere Callahan

The limit superior of a sequence $(a_n)_{n\geq 0}$ is the largest accumulation (or cluster) point of this sequence. An accumulation point is a number c such that in any neighbourhood of c there are infintely many members of the sequence. Analogously, the limit inferior is the least such accumulation point.

If $(a_n)_{n\geq 0}$ is convergent, say with limit a, then
$$\lim_{n\to\infty} {a_n} = \limsup_{n\to\infty}{a_n} = \liminf_{n\to\infty}{a_n} = a$$