Chapter 8: Problem 13
Determine convergence or divergence of the series. $$\sum_{k=3}^{\infty} \frac{e^{1 / k}}{k^{2}}$$
Short Answer
Expert verified
The series \( \sum_{k=3}^{\infty} \frac{e^{1 / k}}{k^{2}} \) is convergent.
Step by step solution
01
Choosing the right test
Let's choose the comparison test: if \( 0 \leq a_n \leq b_n \) for all n greater than some natural number N and \( \sum_{k=3}^{\infty} b_n \) is convergent, then \( \sum_{k=3}^{\infty} a_n \) is also convergent (Direct comparison test).
02
Comparison series
We choose as comparison series \( b_k = \frac{1}{k^2} \). It's clear that \( \frac{e^{1 / k}}{k^{2}} \leq \frac{1}{k^2} \) for \( k \geq 3 \) since \( e^{1 / k} \) is always less than or equal to 1 for \( k \geq 3 \).
03
Evaluate convergence of the comparison series
The comparison series is \( \sum_{k=3}^{\infty} \frac{1}{k^2}\). This series is a p-series with p > 1, so it is convergent.
04
Final conclusion
By the comparison test, since the series \( \sum_{k=3}^{\infty} \frac{1}{k^2} \) which our original series \( \sum_{k=3}^{\infty} \frac{e^{1 / k}}{k^{2}}\) gets compared to, is convergent, our original series is also convergent.
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Comparison Test
When it comes to understanding whether a particular series converges or diverges, the Comparison Test is a reliable tool. This test essentially allows us to determine the behavior of a complex series by comparing it with a simpler series whose convergence properties we already know.
To apply the Comparison Test, you look at two series, say \( a_n \) and \( b_n \) where \( a_n \) is the series in question, and \( b_n \) is the comparison or 'benchmark' series. You need to ensure that the terms of \( a_n \) are always less than or equal to the terms of \( b_n \) after a certain point, typically for all n greater than a specific natural number N. If you find that \( b_n \) converges and has greater or equal terms compared to \( a_n \) beyond that point N, you can conclude that \( a_n \) also converges. This could be seen as a 'guilt by association' in a positive sense - if the 'bigger brother' series is well-behaved (converges), then so is the 'smaller sibling' (our series of interest).
To apply the Comparison Test, you look at two series, say \( a_n \) and \( b_n \) where \( a_n \) is the series in question, and \( b_n \) is the comparison or 'benchmark' series. You need to ensure that the terms of \( a_n \) are always less than or equal to the terms of \( b_n \) after a certain point, typically for all n greater than a specific natural number N. If you find that \( b_n \) converges and has greater or equal terms compared to \( a_n \) beyond that point N, you can conclude that \( a_n \) also converges. This could be seen as a 'guilt by association' in a positive sense - if the 'bigger brother' series is well-behaved (converges), then so is the 'smaller sibling' (our series of interest).
P-Series
The concept of a p-series is fundamental in the study of series convergence. A p-series is presented in the form \( \( \sum_{n=1}^{\infty} \frac{1}{n^p} \) \) with \( p \) being a constant. The convergence or divergence of a p-series is determined purely by the value of \( p \).
A p-series will converge if \( p > 1 \) and diverge if \( p \leq 1 \). It’s interesting to note that this decision rule is fairly straightforward—it’s all about the exponent \( p \) and its relationship to the number 1. This property makes p-series a very handy comparison benchmark when dealing with more complicated series. When you encounter a series where the terms can be bound by the terms of a p-series, you can immediately determine the convergence or divergence by just identifying the relevant \( p \) value. It's this simplicity and clarity that make p-series a popular choice in comparison tests.
A p-series will converge if \( p > 1 \) and diverge if \( p \leq 1 \). It’s interesting to note that this decision rule is fairly straightforward—it’s all about the exponent \( p \) and its relationship to the number 1. This property makes p-series a very handy comparison benchmark when dealing with more complicated series. When you encounter a series where the terms can be bound by the terms of a p-series, you can immediately determine the convergence or divergence by just identifying the relevant \( p \) value. It's this simplicity and clarity that make p-series a popular choice in comparison tests.
Convergent Series
In the realm of mathematics, specifically in the study of infinite series, a 'convergent series' is one where the sum of its terms approaches a finite, specific value as you add more and more terms. In other words, despite adding infinitely many numbers, the total is not unbounded; it's as if the terms become so small in their contribution that the overall sum settles down or 'converges' to a definitive point.
The journey of a convergent series from its first term to infinity is quite fascinating - it gets closer and closer to a certain limit, but never quite overshoots it. When you’re investigating series convergence, you’re essentially checking to see whether this journey leads to a 'mathematical happily ever after' or spirals into an unending 'tale of infinity'. Each test we employ, like the comparison test, is a way to analyze this behavior. And when an intricate series does converge, it means that, while infinity is involved in the process, the outcome is beautifully finite and well-defined.
The journey of a convergent series from its first term to infinity is quite fascinating - it gets closer and closer to a certain limit, but never quite overshoots it. When you’re investigating series convergence, you’re essentially checking to see whether this journey leads to a 'mathematical happily ever after' or spirals into an unending 'tale of infinity'. Each test we employ, like the comparison test, is a way to analyze this behavior. And when an intricate series does converge, it means that, while infinity is involved in the process, the outcome is beautifully finite and well-defined.