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Which of the series, and which diverge? Use any method, and give reasons for your answers. $$\sum_{n=2}^{\infty} \frac{\ln (n+1)}{n+1}$$

Short Answer

Expert verified
The series \( \sum_{n=2}^{\infty} \frac{\ln(n+1)}{n+1} \) diverges.

Step by step solution

01

Understand the Given Series

We are given the series \( \sum_{n=2}^{\infty} \frac{\ln(n+1)}{n+1} \). This series is infinite and starts from \( n = 2 \). We need to determine if it converges or diverges.
02

Choose a Convergence Test

For this problem, the Integral Test is a suitable method for determining convergence. The series terms \( a_n = \frac{\ln(n+1)}{n+1} \) are positive, decreasing, and continuous for \( n \geq 2 \), fulfilling the conditions necessary for the Integral Test.
03

Express the Series as an Integral

Convert the series to an improper integral: \( \int_{2}^{\infty} \frac{\ln(x+1)}{x+1} \, dx \). We will evaluate this integral to determine the convergence of the series.
04

Use Substitution to Simplify the Integral

Use the substitution \( u = x + 1 \), which implies \( du = dx \) and changes the limits of integration from \( x = 2 \) to \( u = 3 \), and as \( x \to \infty \), \( u \to \infty \). The integral becomes \( \int_{3}^{\infty} \frac{\ln(u)}{u} \, du \).
05

Recognize the Divergent Integral

The integral \( \int \frac{\ln(u)}{u} \, du \) is known to diverge. This can be confirmed by noticing that the function \( \ln(u)/u \) mimics the form that typically results in a divergence for integrals.
06

Conclude the Divergence of the Series

Since the integral diverges, the Integral Test tells us that the series \( \sum_{n=2}^{\infty} \frac{\ln(n+1)}{n+1} \) also diverges.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Integral Test
The Integral Test is a powerful tool to determine whether an infinite series converges or diverges. It works by comparing an infinite series with an improper integral of a continuous, positive, and decreasing function. Here's how it works:

  • First, identify the terms of the series, denoted as \( a_n \).
  • Ensure that \( a_n \) is generated from a function \( f(x) \) that satisfies the conditions: it must be continuous, positive, and decreasing for \( x \geq N \), with \( N \) being the lower limit of the series.

Once these conditions are met, integrate the function from an appropriate starting point to infinity. This integral will help you understand the behavior of the series. If the integral converges to a finite value, so does the series. Conversely, if the integral diverges, the series also diverges.

In the example given, the series \( \sum_{n=2}^{\infty} \frac{\ln(n+1)}{n+1} \) utilizes the Integral Test by checking the convergence of \( \int_{2}^{\infty} \frac{\ln(x+1)}{x+1} \ dx \). This improper integral diverges, signaling that the original series diverges as well.
Infinite Series
An infinite series consists of the sum of infinite terms, usually expressed in the form \( \sum_{n=1}^{\infty} a_n \). Such a series can converge to a specific value or diverge, meaning it fails to settle around a specific value.

An infinite series forms an essential component of calculus and mathematical analysis with applications in various fields such as physics, engineering, and computer science. To determine whether an infinite series converges or diverges, one often applies different testing methods. Such methods include Comparison Tests, Ratio Tests, and the Integral Test among others.

In the context of the series \( \sum_{n=2}^{\infty} \frac{\ln(n+1)}{n+1} \), we initially expressed it in summation form and subsequently applied the Integral Test to explore its convergence or divergence. Ultimately, understanding infinite series is crucial for analyzing functions and growth patterns beyond simple arithmetic sums.
Convergence
Convergence in infinite series involves determining if an infinite sequence of numbers has a finite sum. If this sum exists, the series is said to converge. Conversely, if the sum goes to infinity as more terms are added, the series diverges.

Convergence is a critical concept for understanding mathematical functions and analyses that deal with endless sums. These sums can represent power series expansions or solutions to differential equations, among other applications.

To discover convergence, one might use tests like the Integral Test, which checks the integral of a related function to decide the behavior of the infinite series, ensuring that this function always fulfills the conditions needed for the test.

In our example, the series \( \sum_{n=2}^{\infty} \frac{\ln(n+1)}{n+1} \) was found through the Integral Test to diverge. This happens because the integral \( \int_{2}^{\infty} \frac{\ln(x+1)}{x+1} \ dx \) diverges, thereby leading to the conclusion that the sum of the series heads towards infinity.

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Most popular questions from this chapter

Newton's method, applied to a differentiable function \(f(x),\) begins with a starting value \(x_{0}\) and constructs from it a sequence of numbers \(\left\\{x_{n}\right\\}\) that under favorable circumstances converges to a zero of \(f .\) The recursion formula for the sequence is $$x_{n+1}=x_{n}-\frac{f\left(x_{n}\right)}{f^{\prime}\left(x_{n}\right)}$$. a. Show that the recursion formula for \(f(x)=x^{2}-a, a>0\) can be written as \(x_{n+1}=\left(x_{n}+a / x_{n}\right) / 2\) b. Starting with \(x_{0}=1\) and \(a=3,\) calculate successive terms of the sequence until the display begins to repeat. What number is being approximated? Explain.

For what values of \(r\) does the infinite series $$ 1+2 r+r^{2}+2 r^{3}+r^{4}+2 r^{5}+r^{6}+\cdots $$ converge? Find the sum of the series when it converges.

Replace \(x\) by \(-x\) in the Taylor series for \(\ln (1+x)\) to obtain a series for \(\ln (1-x)\). Then subtract this from the Taylor series for \(\ln (1+x)\) to show that for \(|x|<1\) $$\ln \frac{1+x}{1-x}=2\left(x+\frac{x^{3}}{3}+\frac{x^{5}}{5}+\cdots\right)$$

Using a CAS, perform the following steps to aid in answering questions (a) and (b) for the functions and intervals. Step 1: Plot the function over the specified interval. Step 2: Find the Taylor polynomials \(P_{1}(x), P_{2}(x),\) and \(P_{3}(x)\) at \(\bar{x}=0\) Step 3: Calculate the ( \(n+1\) )st derivative \(f^{(n+1)}(c)\) associated with the remainder term for each Taylor polynomial. Plot the derivative as a function of \(c\) over the specified interval and estimate its maximum absolute value, \(M\) Step 4: Calculate the remainder \(R_{n}(x)\) for each polynomial. Using the estimate \(M\) from Step 3 in place of \(f^{(n+1)}(c),\) plot \(R_{n}(x)\) over the specified interval. Then estimate the values of \(x\) that answer question (a). Step 5: Compare your estimated error with the actual error \(E_{n}(x)=\left|f(x)-P_{n}(x)\right|\) by plotting \(E_{n}(x)\) over the specified interval. This will help answer question (b). Step 6: Graph the function and its three Taylor approximations together. Discuss the graphs in relation to the information discovered in Steps 4 and 5. $$f(x)=\frac{1}{\sqrt{1+x}}, \quad|x| \leq \frac{3}{4}$$

a. Series for \(\sinh ^{-1} x \quad\) Find the first four nonzero terms of the Taylor series for $$ \sinh ^{-1} x=\int_{0}^{x} \frac{d t}{\sqrt{1+t^{2}}} $$ b. Use the first three terms of the series in part (a) to estimate \(\sinh ^{-1} 0.25 .\) Give an upper bound for the magnitude of the estimation error.

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