Cauchy sequences


by garyljc
Tags: cauchy, sequences
garyljc
garyljc is offline
#1
Nov5-08, 08:20 AM
P: 106
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 ?
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Office_Shredder
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#2
Nov5-08, 08:43 AM
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a(m) is the same sequence as a(n)
lurflurf
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#3
Nov5-08, 08:47 AM
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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

HallsofIvy
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#4
Nov5-08, 09:49 AM
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Thanks
PF Gold
P: 38,882

Cauchy sequences


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.
garyljc
garyljc is offline
#5
Nov5-08, 11:53 AM
P: 106
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 ?
mathman
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#6
Nov5-08, 02:58 PM
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Quote Quote by garyljc View Post
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 ?
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.
adnan jahan
adnan jahan is offline
#7
Nov10-08, 10:52 PM
P: 95
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
Pere Callahan
Pere Callahan is offline
#8
Nov11-08, 07:29 AM
P: 588
The limit superior of a sequence [itex](a_n)_{n\geq 0}[/itex] 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 [itex](a_n)_{n\geq 0}[/itex] is convergent, say with limit a, then
[tex]
\lim_{n\to\infty} {a_n} = \limsup_{n\to\infty}{a_n} = \liminf_{n\to\infty}{a_n} = a
[/tex]


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