Chapter 9: Problem 34
$$\text { Show that } \sum_{n=1}^{\infty} \frac{1}{\left(\ln \left(2^{n}\right)\right)^{2}} \text { converges. }$$
Short Answer
Expert verified
The series \( \sum_{n=1}^{\infty} \frac{1}{\left(\ln(2^n)\right)^2} \) converges.
Step by step solution
01
Rewrite the series with a simplified expression
First, recognize that the base of the logarithm, \(2^n\), can be rewritten using the logarithmic identity \(\ln(2^n) = n \ln 2\). Therefore, the series \(\sum_{n=1}^{\infty} \frac{1}{(\ln(2^n))^2}\) rewrites as \(\sum_{n=1}^{\infty} \frac{1}{(n \ln 2)^2}\).
02
Simplify the rewritten series
The rewritten series simplifies further as \(\sum_{n=1}^{\infty} \frac{1}{n^2 (\ln 2)^2}\) where \((\ln 2)^2\) is a constant factor that can be factored outside of the summation. This gives us the series \(\frac{1}{(\ln 2)^2} \sum_{n=1}^{\infty} \frac{1}{n^2}\).
03
Test the convergence of the new series
Recognize that the series \(\sum_{n=1}^{\infty} \frac{1}{n^2}\) is a well-known convergent series called the p-series with \(p = 2\). Any p-series with \(p > 1\) converges. Therefore, \(\sum_{n=1}^{\infty} \frac{1}{n^2}\) converges.
04
Consider the constant factor
Since the series \(\sum_{n=1}^{\infty} \frac{1}{n^2}\) is convergent, multiplying it by a constant factor \(\frac{1}{(\ln 2)^2}\) does not affect its convergence. Thus, the series \(\frac{1}{(\ln 2)^2} \sum_{n=1}^{\infty} \frac{1}{n^2}\) also converges.
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Convergent Series
To understand what a convergent series is, it's crucial to start with the concept of a series. A series is the sum of the terms of a sequence of numbers. Convergence, in the context of series, refers to the idea that as you add more terms of the series, the total sum approaches a specific value, called the limit.
For a series to be convergent, this total sum must stabilize and not wander off to infinity. Detecting convergence is vital, especially in mathematical and real-world applications like physics and engineering. These fields require precise calculations with stable outcomes.
For a series to be convergent, this total sum must stabilize and not wander off to infinity. Detecting convergence is vital, especially in mathematical and real-world applications like physics and engineering. These fields require precise calculations with stable outcomes.
- If a series is convergent, there exists a real number limit to which the partial sums of the series approach.
- This concept is opposed to divergence, where the series doesn't settle to any limit.
P-Series
A p-series is a specific type of mathematical series defined by the form \( \sum_{n=1}^{\infty} \frac{1}{n^p} \). Whether a p-series converges or diverges depends on the value of \(p\).
Key facts about p-series:
Key facts about p-series:
- A p-series converges if \(p > 1\).
- It diverges if \(p \leq 1\).
Logarithmic Identity
Logarithmic identities are critical tools in simplifying complex equations into more manageable forms. For this problem, the identity \(\ln(2^n) = n \ln 2\) is used. This transformation is pivotal and is part of a broader family of properties that logarithms possess:
- Logarithms convert multiplicative relationships into additive ones: \(\ln(ab) = \ln a + \ln b\).
- They allow exponentiation to be simplified into multiplication: \(\ln(a^b) = b \ln a\).
Mathematical Proof
A mathematical proof is a logical argument demonstrating the truth of a statement using established mathematical principles. When proving that a series converges, we rely on a sequence of logically connected steps that conform to known theorems and properties.
The process of proving our original problem involves:
The process of proving our original problem involves:
- Recognizing the logarithmic identity to simplify the series expression.
- Identifying the series as a p-series with known convergence behavior.
- Applying the convergence criteria of p-series.
Infinite Series
Infinite series are sums that extend indefinitely, often written as \(\sum_{n=1}^{\infty} a_n\). They are foundational in mathematical analysis and have diverse applications across various scientific fields.
When working with infinite series:
When working with infinite series:
- It's critical to determine whether they converge to a limit.
- Various tests and techniques, such as the p-series test, can be employed to evaluate convergence.