Chapter 5: Problem 233
Does \(\sum_{n=1}^{\infty}(\ln n)^{-\ln n}\) converge? (Hint: Use \(t=e^{\ln (t)}\) to compare to a \(p\) - series.)
Short Answer
Expert verified
The series \( \sum_{n=1}^{\infty} (\ln n)^{-\ln n} \) converges by comparison to a \( p \)-series.
Step by step solution
01
Understand the Series and Use the Hint
The given series is \( \sum_{n=1}^{\infty} (\ln n)^{-\ln n} \). The hint suggests rewriting the series term: \((\ln n)^{-\ln n} = e^{-\ln n \cdot \ln(\ln n)}\). This uses the identity \(a^{-b} = e^{-b \ln a}\).
02
Express the Term More Clearly
We have that \( (\ln n)^{-\ln n} = e^{- (\ln n)( \ln(\ln n))}\). This form helps in analyzing the asymptotic behavior as \( n \) approaches infinity.
03
Compare to a Known Convergent Series
This expression \( e^{- (\ln n)( \ln(\ln n))} \) suggests rapid decay as \( n \to \infty\). These terms decrease faster than any negative power of \( n \) or its logarithm. Since \((\ln n)^{\ln n}\) increases rapidly, \( e^{- (\ln n)( \ln(\ln n))} \) decreases quickly as well, faster than any \( n^{-p} \) series with positive \( p \).
04
Determine Convergence
The series terms \((\ln n)^{- \ln n}\) decrease very rapidly. For very large \( n \), \( (\ln n)( \ln(\ln n)) \) becomes large, making \( e^{- (\ln n)( \ln(\ln n))} \) very small. Since the terms decrease faster than any power of a logarithm, it suggests absolute convergence.
05
Conclusion Using Comparison Test
The given series \( \sum_{n=1}^{\infty}(\ln n)^{-\ln n} \), can be compared to \( \sum 1/n^p \), which converges for \( p > 1 \). By comparison, since our series terms go to zero faster than \( n^{-p} \), the series 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 handy tool in determining the convergence of infinite series. This test allows us to deduce the convergence of a series by comparing it with another series whose convergence behavior is already known. Here's how it works:
This approach provides a straightforward pathway to decide on convergence by setting a simpler, familiar series as a benchmark.
- If you have two series, say \(\sum a_n\) and \(\sum b_n\), and 0 ≤ a_n ≤ b_n for all n, then 👉 if \(\sum b_n\) converges, so does \(\sum a_n\).
- Conversely, if \(\sum a_n\) diverges, so does \(\sum b_n\).
This approach provides a straightforward pathway to decide on convergence by setting a simpler, familiar series as a benchmark.
Exponential Decay
Exponential decay refers to the decrease in a quantity by a consistent percentage rate over time. In the context of our series, we see exponential decay as \(n\) increases. The expression \(e^{-(\ln n)(\ln(\ln n))}\) reveals how the terms diminish as \(n\) gains larger values.
Compare this to simpler exponential decay, like cutting an amount by half repeatedly:
Compare this to simpler exponential decay, like cutting an amount by half repeatedly:
- Our term involves a complex exponent \(\ln n \, \ln(\ln n)\), with both logarithmic terms increasing with \(n\).
- The rapid growth of this exponent means the base of the exponent, \(e\), will shrink the term to practically zero very quickly.
p-Series
A p-series is a kind of series that takes the form \(\sum \frac{1}{n^p}\) where \(p\) is a positive constant. These series are well-explored, with clear rules:
That's the brilliance of p-series. They serve as an apt litmus test for series that may not have \(1/n^p\) structures but behave similarly in growth or decay patterns. Always paying attention to these comparisons helps tackle convergence with tried-and-tested methods.
- The p-series converges if \(p > 1\).
- It diverges if \(p \leq 1\).
That's the brilliance of p-series. They serve as an apt litmus test for series that may not have \(1/n^p\) structures but behave similarly in growth or decay patterns. Always paying attention to these comparisons helps tackle convergence with tried-and-tested methods.
Logarithmic Functions
Logarithmic functions like \(\ln x\) play a critical role in analyzing our series term \(e^{-(\ln n)(\ln(\ln n))}\). These functions help capture how terms increase or decrease as \(n\) becomes very large.
- Functions of the form \(\ln n\) increase, but very slowly, indicating that even when \(n\) is astronomical, \(\ln n\) remains manageable.
- Complex logarithmic expressions like \(\ln(\ln n)\) increase even more slowly, allowing the series analysis to tackle these growths methodically.