Convergence or divergence (series)

In summary, the conversation discusses the use of the Alternating Series Test to determine the convergence of the series Ʃ[(-1)^n (cosn)^2]/√n. The first step is to find the limit and test if it is zero, followed by checking if a_{n+1} is less than or equal to a_n. The initial value for n is typically not relevant, but is included in the proper notation when writing the series.
  • #1
bfusco
128
1

Homework Statement


Ʃ[(-1)^n (cosn)^2]/√n

The Attempt at a Solution


i don't have the slightest clue where to start
 
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  • #2
Since this is a series and there is an alternating sign in the consecutive partial sums, you should use the Alternating Series Test.
[tex]\sum^{\infty}_{n=?}(-1)^n \frac{(\cos n)^2}{√n} [/tex]
You have not stated the initial value for n.

The first step: Let [itex]a_n=\frac{(\cos n)^2}{√n}[/itex]. Find the limit and test if it's zero.
Second step: Is [itex]a_{n+1}\leq a_n[/itex]?
If both of these conditions are satisfied, then the series converges.
 
Last edited:
  • #3
The initial value for n doesn't matter. It's presumably not zero.
 
  • #4
JG89 said:
The initial value for n doesn't matter. It's presumably not zero.

The initial value of n is normally ignored, but stating the latter forms part of the proper notation when writing the series with the summation symbol.
 

FAQ: Convergence or divergence (series)

1. What is the difference between convergence and divergence?

Convergence and divergence are terms used in mathematics to describe the behavior of a series. A series is said to converge if the terms in the series approach a finite limit as the number of terms increases, and it is said to diverge if the terms in the series do not approach a finite limit.

2. How can I determine if a series converges or diverges?

There are several tests that can be used to determine the convergence or divergence of a series. These include the ratio test, the comparison test, the integral test, and the root test. Each test has its own conditions and limitations, so it is important to understand the requirements for each test before using them.

3. What is the importance of convergence and divergence in mathematics?

Convergence and divergence are important concepts in mathematics because they allow us to understand the behavior of infinite series. They are used in various mathematical applications, such as in calculus, probability, and statistics, and play a crucial role in understanding the convergence of numerical methods used in solving equations.

4. Can a series be both convergent and divergent?

No, a series can either be convergent or divergent, but not both. If a series converges, it means that the terms in the series approach a finite limit, and if a series diverges, it means that the terms in the series do not approach a finite limit.

5. What is the difference between absolute convergence and conditional convergence?

Absolute convergence and conditional convergence are two types of convergence that apply to series with both positive and negative terms. Absolute convergence occurs when the absolute values of the terms in the series converge, while conditional convergence occurs when the series converges, but the absolute values of the terms do not converge.

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