How to prove that this series bounded and monotonic

In summary, the conversation discusses proving the monotonicity and boundedness of the sequence Xn=(1-1/2)(1-1/4)..(1-(1/(2^n))). It is shown that the sequence is monotonic by demonstrating that x_n > x_{n+1} for all n. It is also mentioned that the sequence is bounded, with x_1=1/2 being an upper bound. The sequence is described as decreasing and composed of fractions that are getting smaller and smaller.
  • #1
transgalactic
1,395
0
Xn=(1-1/2)(1-1/4)..(1-(1/(2^n))

i tried to prove that its monotonic
by :
1-1/(2^n) = (2^n-1)/2^n

2^n -1 <2^n
obviously its correct
the numerator of each object is smaller then the denominator.

what now??

and how to prove that its bounded?
 
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  • #2
You need to show that [tex]x_n > x_{n+1}[/tex] for all n given that [tex]x_n(1-2^{-n}) = x_{n+1}[/tex]

It should be fairly easy from here
 
  • #3
that proves that its monotonic
how to prove that its bounded?
 
  • #4
Is it increasing or decreasing?
 
  • #5
each next member is bigger then the previous one
so its increasing

1-1/2 1-1/4 1-1/8 etc..
 
  • #6
That's now what the sequence is. The sequence is

1/2, 1/2*3/4, 1/2*3/4*7/8 etc.

you should be able to see this from how xn is defined.
 
  • #7
ok so it getting smaller and smaller
how to prove that its bounded?
 
  • #8
There's a pretty obvious lower bound. And since it is decreasing, isn't x1= 1/2 an upper bound?
 

Related to How to prove that this series bounded and monotonic

1. How do you determine if a series is bounded?

A series is considered bounded if the absolute value of each term in the series is less than or equal to a constant value, regardless of the number of terms in the series. This means that the series does not increase or decrease without bound as the number of terms increases.

2. What is the definition of a monotonic series?

A monotonic series is a series where the terms either consistently increase or consistently decrease. This means that the series has a consistent trend and does not have any large fluctuations or changes in direction.

3. How do you prove that a series is bounded and monotonic?

To prove that a series is bounded and monotonic, you must show that the absolute value of each term in the series is less than or equal to a constant value, and that the terms either consistently increase or consistently decrease. This can be done through mathematical analysis, such as using the limit comparison test or the ratio test.

4. Why is it important to prove that a series is bounded and monotonic?

It is important to prove that a series is bounded and monotonic because it ensures that the series is convergent. A convergent series has a finite limit, meaning that the series will eventually reach a fixed value. This is important in many areas of science, such as in physics and economics.

5. Are there any common mistakes when trying to prove that a series is bounded and monotonic?

One common mistake when trying to prove that a series is bounded and monotonic is assuming that the series is bounded and monotonic without proper mathematical analysis. It is important to carefully examine the series and use appropriate tests and techniques to prove that it is indeed bounded and monotonic. Additionally, it is important to remember that proving a series is bounded and monotonic does not necessarily mean it is convergent, as there are other factors that can affect convergence.

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