Chapter 11: Problem 25
Determine whether the series is convergent or divergent. $$ \sum_{n=1}^{\infty} \frac{1}{n^{2}+n^{3}} $$
Short Answer
Expert verified
The series is convergent by the Comparison Test.
Step by step solution
01
Analyze the General Term
The general term of the series is \(a_n = \frac{1}{n^2 + n^3} = \frac{1}{n^2(1+n)}\). This transformation helps us in comparing this series with others and applying known convergence tests.
02
Apply the Comparison Test
Consider the function \(b_n = \frac{1}{n^3}\), which is known to be a convergent \(p\)-series with \(p = 3 > 1\). Notice that \(a_n = \frac{1}{n^2(1+n)} < \frac{1}{n^3} = b_n\) for all \(n > 1\).
03
Check the Limits for Comparison Test
Since we have \(a_n < b_n\) and \(\sum_{n=1}^{\infty} b_n\) is convergent, by the Comparison Test, we conclude that \(\sum_{n=1}^{\infty} a_n\) is also convergent. Thus, \(\sum_{n=1}^{\infty} \frac{1}{n^2 + n^3}\) converges.
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Comparison Test
The Comparison Test is a crucial method for determining the convergence or divergence of an infinite series. It's a powerful tool in series analysis because it allows you to infer the behavior of a complicated series by comparing it with another series whose convergence properties are already known. When you have a series \(\sum a_n\), the idea is to find another series \(\sum b_n\) that is easier to understand:
- If \(0 \leq a_n \leq b_n\) for all \(n\) beyond some point, and \(\sum b_n\) is convergent, then \(\sum a_n\) is also convergent.
- Conversely, if \(a_n \geq b_n \geq 0\) and \(\sum b_n\) diverges, then \(\sum a_n\) diverges.
P-Series
A p-series is a specific type of series that takes the form \(\sum \frac{1}{n^p}\), where \(p\) is a positive constant. The convergence of a p-series largely depends on the value of \(p\).
- If \(p > 1\), the p-series converges.
- If \(p \leq 1\), the p-series diverges.
Infinite Series
An infinite series can be thought of as the sum of an infinite sequence of terms. Mathematically, this is expressed as \(\sum_{n=1}^{\infty} a_n\). Whether this sum converges (results in a finite number) or diverges (grows indefinitely or oscillates) depends on the behavior of its terms as \(n\) approaches infinity. Understanding this behavior is essential in establishing the convergence criteria.
- If the series converges, the partial sums \(S_n = a_1 + a_2 + ... + a_n\) approach a finite limit as \(n\) goes to infinity.
- Conversely, if \(S_n\) does not approach a finite limit, the series diverges.
Series Analysis
Series analysis involves a set of techniques and methodologies that help in the study and understanding of series' behavior. It encapsulates the exploration of whether a series converges or diverges, and under what conditions.The task in our example was to explore the convergence of \(\sum \frac{1}{n^2+n^3}\). We began by simplifying the general term \(a_n = \frac{1}{n^2(1+n)}\) and then employed the Comparison Test, looking to compare our series with a known simpler series. During series analysis, the choice of tests, such as the Comparison Test, depends on its suitability and the nature of the series.
- Choose the Series you're comparing to wisely. Some series, like p-series or geometric series, have well-understood properties that can be leveraged effectively.
- Breaking down the series' terms often helps to see patterns and relationships that are less visible at first glance.