Proving a monotonic sequence is unbounded

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In summary, the conversation discusses how to prove that a given sequence of real numbers is unbounded. The idea of using contradiction is suggested, but the speaker is unsure of how to proceed. The conversation then delves into the concept of convergence and how it relates to the sequence in question.
  • #1
alligatorman
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I'm trying to prove that the sequence
[tex]x_1,x_2,_\cdots[/tex]
of real numbers, where
[tex]x_1=1[/tex] and [tex]x_{n+1}=x_n+\frac{1}{x_n^2}[/tex] for each [tex]n=1,2, \cdots[/tex]

is unbounded.

(sorry for the ugly latex! i don't know if there's a way to format that better)

I'm thinking of proving by contradiction, assuming it is bounded and then somehow getting it to imply that the sequence is not increasing, but I'm not sure how to go about it.

Any hints?
 
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  • #2
If it were bounded, it would converge.
 
  • #3
And if it were to converge to, say, x, that limit would satisfy
[tex]x= x+ \frac{1}{x}[/tex]
What values of x satisfy that?
 
  • #4
HallsofIvy said:
And if it were to converge to, say, x, that limit would satisfy
[tex]x= x+ \frac{1}{x}[/tex]
What values of x satisfy that?

:confused: I don't understand why this is true.
 
  • #5
Start with [itex]x_{n+1}= x_n+ 1/x_n[/itex] and take the limit, as n goes to infinity ,of both sides. If the sequence [itex]{x_n}[/itex] converges to some number, x, then each "[itex]x_n[/itex]" or "[itex]x{n+1}[/itex]" term will go to x.
 

FAQ: Proving a monotonic sequence is unbounded

What is a monotonic sequence?

A monotonic sequence is a sequence of numbers that either always increases or always decreases. This means that each term in the sequence is either larger than or equal to the previous term (in an increasing sequence) or smaller than or equal to the previous term (in a decreasing sequence).

Why is it important to prove that a monotonic sequence is unbounded?

Proving that a monotonic sequence is unbounded is important because it helps us understand the behavior of the sequence. If a sequence is unbounded, it means that the terms in the sequence continue to get larger (in an increasing sequence) or smaller (in a decreasing sequence) without limit. This information can be useful in various mathematical and scientific applications.

What is the process for proving that a monotonic sequence is unbounded?

The process for proving that a monotonic sequence is unbounded involves showing that there is no upper or lower limit to the terms in the sequence. This can be done by using the definition of a monotonic sequence and assuming that there is a limit, then showing that this assumption leads to a contradiction. This proves that the sequence is unbounded.

Can a monotonic sequence be both bounded and unbounded?

No, a monotonic sequence cannot be both bounded and unbounded. A sequence can either have an upper and/or lower limit, making it bounded, or it can have no limit at all, making it unbounded. A monotonic sequence by definition cannot have both properties.

How does proving a monotonic sequence is unbounded relate to the concept of limits?

Proving a monotonic sequence is unbounded is all about understanding the behavior of the terms in the sequence as they approach a limit. By showing that there is no limit to the terms in the sequence, we are essentially proving that the sequence does not have a limit. This helps us understand the behavior of the sequence and its relationship to the concept of limits.

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