Prove the sequence n/2^n converges to 0

  • Thread starter naele
  • Start date
  • Tags
    Sequence
In summary, the given problem seeks to prove that the sequence x_n=\frac{n}{2^n} converges to 0. The suggested approach is to use the squeeze theorem, with the sequence \frac{1}{n} < \frac{n}{2^n} < 0 and show that it is bounded below by 0 and monotonically decreasing. However, this is not enough to prove convergence, as shown by the example of 1+1/n. Another approach suggested is to compare the sequence to r^n, where r<1, but this requires showing that x_{n+1}/x_n has a limit less than 1.
  • #1
naele
202
1

Homework Statement


Show that [tex]x_n=\frac{n}{2^n}[/tex] converges to 0

Homework Equations


Squeeze theorem?


The Attempt at a Solution


I've already proven that for [tex]n\geq 4, n^2 < 2^n[/tex] which means that [tex]\frac{1}{n} < \frac{n}{2^n}[/tex]. My desired approach is to use the squeeze theorem, but I cannot think of another sequence greater than n/2^n that would converge to 0.
 
Physics news on Phys.org
  • #2
Can you show that (n + 1)/2n + 1 < n/2n for all n past some initial value? (Proof by induction?) If you can show this, and then use the fact that the sequence is bounded below (by zero), you have sequence that is monotonically decreasing and bounded below by 0, hence converges to 0.
 
  • #3
You got the sign wrong. [tex] \frac{1}{n} > \frac{n}{2^n} > 0 [/tex] for n>4. Next, apply the squeeze theorem.
you have sequence that is monotonically decreasing and bounded below by 0, hence converges to 0.

that's not enough. For example, 1+1/n is monotonically decreasing and it's bounded below by 0, but it does not converge to 0.
 
  • #4
hamster143 said:
that's not enough. For example, 1+1/n is monotonically decreasing and it's bounded below by 0, but it does not converge to 0.
You got me. Mea culpa.
 
  • #5
Thanks hamster, my teacher warned me against this kind of mistake too. Live and learn I guess :)
 
  • #6
If [itex]x_{n+1}/x_n[/itex] has limit r< 1 then you can "compare" the sequence to [itex]r^n[/itex].
 

What is the sequence n/2^n?

The sequence n/2^n is a mathematical sequence where each term is obtained by dividing n (a positive integer) by 2^n (2 raised to the power of n). It can be written as: 1/2, 2/4, 3/8, 4/16, 5/32, and so on.

What does it mean for a sequence to converge?

A sequence converges when its terms get closer and closer to a single number as you go further down the sequence. In other words, the difference between each term and the limit (the number the sequence is approaching) becomes smaller and smaller.

How do you prove that a sequence converges?

To prove that a sequence converges, you need to show that the terms of the sequence get closer and closer to a specific number (the limit) as you go further down the sequence. This can be done through various mathematical methods such as the squeeze theorem, the ratio test, or the limit comparison test.

Why is it important to prove that a sequence converges?

Proving that a sequence converges is important because it allows us to make accurate predictions about the behavior of the sequence in the long run. It also helps us understand the underlying patterns and relationships within the sequence, which can be useful in various fields of science and mathematics.

How can you prove that the sequence n/2^n converges to 0?

To prove that the sequence n/2^n converges to 0, we can use the limit comparison test. By comparing it to the sequence 1/2^n, which is known to converge to 0, we can show that the two sequences have the same behavior and thus, n/2^n also converges to 0. This can also be proven using the ratio test, where we can show that the limit of n/2^n as n approaches infinity is equal to 0.

Similar threads

  • Calculus and Beyond Homework Help
Replies
1
Views
255
  • Calculus and Beyond Homework Help
Replies
4
Views
306
  • Calculus and Beyond Homework Help
Replies
17
Views
1K
  • Calculus and Beyond Homework Help
Replies
34
Views
2K
  • Calculus and Beyond Homework Help
Replies
13
Views
964
  • Calculus and Beyond Homework Help
Replies
4
Views
881
  • Calculus and Beyond Homework Help
Replies
3
Views
413
  • Calculus and Beyond Homework Help
Replies
6
Views
387
  • Calculus and Beyond Homework Help
Replies
3
Views
549
  • Calculus and Beyond Homework Help
Replies
9
Views
1K
Back
Top