Divergence of a sequence


by Bipolarity
Tags: divergence, sequence
Bipolarity
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#1
Nov8-12, 06:26 PM
P: 783
I'm trying to understand divergence of a sequence (not series). What methods can I use to prove divergence? I know that convergence can be proven using various methods, such as squeeze theorem and sum, difference, product and quotient rule etc.

Could I use the following to prove divergence?

If [itex] a_{n} [/itex] is a sequence of real numbers, [itex] f(n) = a_{n} [/itex] and [itex] \lim_{n→∞} f(n) [/itex] does not exist, but is not equal to ∞ or -∞, does [itex] a_{n} [/itex] necessarily diverge?

If [itex] a_{n} [/itex] is a sequence of real numbers, [itex] f(n) = a_{n} [/itex] and [itex] \lim_{n→∞} f(n) = ∞ [/itex], does [itex] a_{n} [/itex] necessarily diverge?

These two ideas will greatly facilitate my understanding of sequence divergence.
Thanks!

BiP
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micromass
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#2
Nov8-12, 06:40 PM
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Yes to both questions.
Bipolarity
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#3
Nov8-12, 08:24 PM
P: 783
Quote Quote by micromass View Post
Yes to both questions.
Hey micro, but what about the sequence [itex] a_{n} = sin(2πn) [/itex]. It is the case that
[itex] \lim_{n→∞}f(n) [/itex] does not exist, yet the limit of [itex]a_{n}[/itex] converges to 0, right??

BiP

micromass
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#4
Nov8-12, 08:36 PM
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Divergence of a sequence


The limit [itex]\lim_{n\rightarrow +\infty} f(n)[/itex] does exist and is zero. (I assume that n is always an integer)

However, if you extend f to [itex]f(x)=\sin(2\pi x)[/itex] for [itex]x\in\mathbb{R}[/itex], then the limit [itex]\lim_{x\rightarrow +\infty} f(x)[/itex] doesn't exist.
pwsnafu
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#5
Nov8-12, 08:37 PM
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Quote Quote by Bipolarity View Post
[itex] \lim_{n→∞}f(n) [/itex] does not exist
Why do you say that?

Edit: ninjaed
Bipolarity
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#6
Nov8-12, 10:14 PM
P: 783
Quote Quote by micromass View Post
The limit [itex]\lim_{n\rightarrow +\infty} f(n)[/itex] does exist and is zero. (I assume that n is always an integer)

However, if you extend f to [itex]f(x)=\sin(2\pi x)[/itex] for [itex]x\in\mathbb{R}[/itex], then the limit [itex]\lim_{x\rightarrow +\infty} f(x)[/itex] doesn't exist.
micromass, I'm sorry I think I misphrased my question. When I refer to f(n) in my original post, I refer to it as a function with domain ℝ as opposed to [itex]a_{n}[/itex] which I take to be defined only for natural numbers.

Given this clarification, which of the following original statements is true and why?

BiP


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