Proving Converging Sequences: {an}, {an + bn}, {bn}

In summary, the proof or counterexample for the statement "If {an} and {an + bn} are convergent sequences, then {bn} is a convergent sequence" is being discussed. The suggested approach is to use delta epsilon definitions, with the hint that the limit of a sum is equal to the sum of the limits. The conversation also mentions a possible counterexample and the use of a trick involving bn=(an+bn)+(-an).
  • #1
Nan1teZ
11
0

Homework Statement



Prove or give a counterexample: If {an} and {an + bn} are convergent sequences, then {bn} is a convergent sequence.


2. The attempt at a solution

Ok I couldn't think of any counterexamples, so I tried to prove it using delta epsilon definitions:

|an - L| < E
|an + bn - M| < E
want to show: |bn - N| < E

Is this the right approach?
 
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  • #2
Yes, and M=L+N, right? You can certainly prove that. It's the correct approach.
 
  • #3
yeah I got the N = M-L part. But then after that I go in circles trying to show it is < Epsilon. =[

What's the little trick?
 
  • #4
Hint: [tex] \lim_{n \to \infty} \left\{ a_{n} + b_{n} \right\} = \lim_{n \to \infty} a_{n} + \lim_{n \to \infty} b_{n}[/tex]
 
  • #5
And bn=(an+bn)+(-an).
 

Related to Proving Converging Sequences: {an}, {an + bn}, {bn}

1. What is a converging sequence?

A converging sequence is a sequence of numbers that approaches a specific value as the number of terms increases. In other words, as more terms are added to the sequence, the numbers get closer and closer to a fixed number, known as the limit.

2. How do you prove that a sequence is converging?

To prove that a sequence is converging, you must show that the terms of the sequence get closer and closer to the limit as the number of terms increases. This can be done by using mathematical techniques such as the epsilon-delta definition or the squeeze theorem.

3. What is the difference between {an} and {an + bn}?

{an} is a sequence where each term is multiplied by a constant value, while {an + bn} is a sequence where each term is added to another sequence, {bn}. This means that {an + bn} is a combination of two sequences, while {an} is a single sequence.

4. How does the value of {bn} affect the convergence of {an + bn}?

The value of {bn} can either help or hinder the convergence of {an + bn}, depending on its behavior. If {bn} is a converging sequence with a limit of 0, then {an + bn} will also converge to the same limit. However, if {bn} is a diverging sequence, then {an + bn} may also diverge or oscillate.

5. Can a sequence be converging and diverging at the same time?

No, a sequence cannot be both converging and diverging at the same time. A sequence can either converge to a specific limit or diverge to positive or negative infinity. However, it is possible for a sequence to appear to be converging but actually be oscillating between two or more values, in which case it is not truly converging.

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