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17Calculus Infinite Series - p-Series

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p-Series Convergence Theorem

\(\displaystyle{\sum_{n=1}^{\infty}{\frac{1}{n^p}} = 1 + \frac{1}{2^p} + \frac{1}{3^p} + \frac{1}{4^p} + ~~ . . . }\)

\( 0 \lt p \leq 1\)

\( p>1\)

diverges

converges

Note - In the proof section below, we show a more efficient way to write this theorem statement.

The p-series is a pretty straight-forward series to understand and use. One detail you need to notice is that the theorem covers cases only where \( p > 0\).

Question: Do you know what happens when \( p \leq 0\)?

Okay, so testing for convergence and divergence of p-series looks pretty easy and, fortunately, it is. But it is also very powerful. To help you cement the idea more firmly in your mind, read through the panel below discussing the proof of this test. You do not have to understand the integral test at this point to understand this proof.

p-Series Convergence Theorem Proof

p-Series Convergence Theorem

The p-series \(\displaystyle{\sum_{n=1}^{\infty}{\frac{1}{n^p}}}\)

diverges

converges

\( 0 \lt p \leq 1\)

\( p>1\)

To prove this, we will use the Integral Test and the Special Improper Integral.

The integral test tells us that we can set up the integral \(\displaystyle{ \int_1^{\infty}{\frac{1}{x^p} dx} }\)

If this integral converges, then so does the series. Similarly, if the integral diverges, the series also diverges.

We will look at these five cases.
1. \( p > 1 \)
2. \( p = 1 \)
3. \( 0 \lt p \leq 1\)
4. \( p = 0 \)
5. \( p \lt 0 \)
Note: Although the last two cases are not part of the theorem, we will show what happens in those two cases to answer the question at the top of the page and for completeness.

Let's use the Integral Test to prove convergence and divergence by calculating the corresponding improper integrals. For most of the cases, we need to set up a limit as follows

\(\displaystyle{ \lim_{b \to \infty}{ \int_1^{b}{\frac{1}{x^p} dx}} }\)

Since the function \(\displaystyle{ \frac{1}{x^p} }\) is continuous on the interval \( [1,b] \), this integral can be evaluated. So, now let's look at each case individually.

Case 1: \( p > 1 \)
When \( p > 1 \), the integral is continuous and decreasing on the interval. So the Integral Test applies.

\(\displaystyle{ \lim_{b \to \infty}{ \int_1^{b}{\frac{1}{x^p} dx}} }\)

\(\displaystyle{ \lim_{b \to \infty}{ \int_1^{b}{ x^{-p} dx}} }\)

\(\displaystyle{ \lim_{b \to \infty}{ \left[ \frac{x^{-p+1}}{-p+1} \right]_{1}^{b} } }\)

\(\displaystyle{ \frac{1}{-p+1} \lim_{b \to \infty}{\left[ b^{-p+1} - 1^{-p+1} \right]} }\)

\(\displaystyle{ \frac{1}{-p+1} [ 0 - 1 ] = \frac{1}{p-1} }\)

Since \(\displaystyle{ \frac{1}{p-1} }\) is finite, the integral converges and, therefore, by the Integral Test, the series also converges.

Case 2: \( p=1 \)
When \(p = 1\), we have

\(\displaystyle{ \lim_{b \to \infty}{ \int_1^{b}{\frac{1}{x} dx}} \to \lim_{b \to \infty }{ \left[ \ln(x) \right]_{1}^{b} } \to \lim_{b \to \infty }{ \ln(b) } - 0 = \infty }\)
Since the limit is infinity, the series diverges.

Case 3: \( 0 \lt p \lt 1\)
We will use the Integral Test again.

\(\displaystyle{ \lim_{b \to \infty}{ \int_1^{b}{\frac{1}{x^p} dx}} }\)

\(\displaystyle{ \lim_{b \to \infty}{ \int_1^{b}{ x^{-p} dx}} }\)

\(\displaystyle{ \lim_{b \to \infty}{ \left[ \frac{x^{-p+1}}{-p+1} \right]_{1}^{b} } }\)

\(\displaystyle{ \frac{1}{-p+1} \lim_{b \to \infty}{\left[ b^{-p+1} - 1^{-p+1} \right]} }\)

So far in these calculations, we have the same equation as we did in case 1. However, in this case, \( 0 \lt p \lt 1\), which means that the exponent \(-p+1 > 0\) and therefore \(\displaystyle{ \lim_{b \to \infty}{ b^{-p+1} } \to \infty }\). So the entire integral diverges, which means that the series diverges.

Case 4: \( p = 0 \)
When \( p = 0 \), the series is \(\displaystyle{ \sum_{n=1}^{\infty}{1} }\). Since \(\displaystyle{ \lim_{n \to \infty}{1} = 1 \neq 0 }\) the divergence test tells us that the series diverges.

Case 5: \( p \lt 0 \)
We can write the fraction \(\displaystyle{ \frac{1}{n^p} = n^{-p} }\). Since \( p \lt 0 \), the exponent here is positive and so the terms are increasing. Since \(\displaystyle{ \lim_{n \to \infty}{ \frac{1}{n^p} } \neq 0 }\), the series diverges by the divergence test.

In Summary: For the series \(\displaystyle{\sum_{n=1}^{\infty}{\frac{1}{n^p}}}\)
1. \( p > 1 \) converges by the integral test
2. \( p = 1 \) diverges by the integral test
3. \( 0 \lt p \lt 1\) diverges by the integral test
4. \( p = 0 \) diverges by the divergence test
5. \( p \lt 0 \) diverges by the divergence test

- qed -

Note: We have also seen the p-series theorem where, instead of writing \( 0 \lt p \leq 1\) for where the series diverges, it says the series diverges for \( p \leq 1 \). Of course, this includes all of the cases above. This is a more efficient way to write the theorem statement.

Euler's Constant

Here is a great video discussing one use of a p-series.
Note - - Euler's Constant is not the same as Euler's Number.

PatrickJMT - Euler's Constant [16min-10secs]

video by PatrickJMT

Practice

Unless otherwise instructed, determine whether these series converge or diverge using the p-series convergence theorem.

\(\displaystyle{ \sum_{n=1}^{\infty}{ \frac{2}{n^2} } }\)

Problem Statement

Determine whether the series \(\displaystyle{ \sum_{n=1}^{\infty}{ \frac{2}{n^2} } }\) converges or diverges using the p-series convergence theorem.

Solution

3163 video

close solution

\(\displaystyle{ \sum_{n=1}^{\infty}{ \frac{1}{n^{1.4}} } }\)

Problem Statement

Determine whether the series \(\displaystyle{ \sum_{n=1}^{\infty}{ \frac{1}{n^{1.4}} } }\) converges or diverges using the p-series convergence theorem.

Final Answer

The series \(\displaystyle{ \sum_{n=1}^{\infty}{ \frac{1}{n^{1.4}} } }\) is a convergent p-series.

Problem Statement

Determine whether the series \(\displaystyle{ \sum_{n=1}^{\infty}{ \frac{1}{n^{1.4}} } }\) converges or diverges using the p-series convergence theorem.

Solution

253 video

video by PatrickJMT

Final Answer

The series \(\displaystyle{ \sum_{n=1}^{\infty}{ \frac{1}{n^{1.4}} } }\) is a convergent p-series.

close solution

\(\displaystyle{ \sum_{n=1}^{\infty}{ \frac{1}{n^{0.7}} } }\)

Problem Statement

Determine whether the series \(\displaystyle{ \sum_{n=1}^{\infty}{ \frac{1}{n^{0.7}} } }\) converges or diverges using the p-series convergence theorem.

Final Answer

The series \(\displaystyle{ \sum_{n=1}^{\infty}{ \frac{1}{n^{0.7}} } }\) is a divergent p-series.

Problem Statement

Determine whether the series \(\displaystyle{ \sum_{n=1}^{\infty}{ \frac{1}{n^{0.7}} } }\) converges or diverges using the p-series convergence theorem.

Solution

254 video

video by PatrickJMT

Final Answer

The series \(\displaystyle{ \sum_{n=1}^{\infty}{ \frac{1}{n^{0.7}} } }\) is a divergent p-series.

close solution

\(\displaystyle{ \sum_{n=1}^{\infty}{ n^{-\pi} } }\)

Problem Statement

Determine whether the series \(\displaystyle{ \sum_{n=1}^{\infty}{ n^{-\pi} } }\) converges or diverges using the p-series convergence theorem.

Solution

3153 video

close solution

\(\displaystyle{ \sum_{n=1}^{\infty}{ (n^{-2.4} + 8n^{-1.6}) } }\)

Problem Statement

Determine whether the series \(\displaystyle{ \sum_{n=1}^{\infty}{ (n^{-2.4} + 8n^{-1.6}) } }\) converges or diverges using the p-series convergence theorem.

Solution

3154 video

close solution

\(\displaystyle{ 1 + \frac{1}{\sqrt{2}} + \frac{1}{\sqrt{3}} + \frac{1}{\sqrt{4}} + . . . }\)

Problem Statement

Determine whether the series \(\displaystyle{ 1 + \frac{1}{\sqrt{2}} + \frac{1}{\sqrt{3}} + \frac{1}{\sqrt{4}} + . . . }\) converges or diverges using the p-series convergence theorem.

Solution

This problem is solved by two different instructors.

3155 video

3155 video

close solution

\(\displaystyle{ 1 + \frac{1}{\sqrt[3]{4}} + \frac{1}{\sqrt[3]{9}} + \frac{1}{\sqrt[3]{16}} + . . . }\)

Problem Statement

Determine whether the series \(\displaystyle{ 1 + \frac{1}{\sqrt[3]{4}} + \frac{1}{\sqrt[3]{9}} + \frac{1}{\sqrt[3]{16}} + . . . }\) converges or diverges using the p-series convergence theorem.

Solution

3156 video

close solution

\(\displaystyle{ 1+ \frac{1}{2\sqrt[3]{2}} + \frac{1}{3\sqrt[3]{3}} + \frac{1}{4\sqrt[3]{4}} + . . . }\)

Problem Statement

Determine whether the series \(\displaystyle{ 1+ \frac{1}{2\sqrt[3]{2}} + \frac{1}{3\sqrt[3]{3}} + \frac{1}{4\sqrt[3]{4}} + . . . }\) converges or diverges using the p-series convergence theorem.

Final Answer

The series \(\displaystyle{ 1+ \frac{1}{2\sqrt[3]{2}} + \frac{1}{3\sqrt[3]{3}} + \frac{1}{4\sqrt[3]{4}} + . . . }\) is a convergent p-series.

Problem Statement

Determine whether the series \(\displaystyle{ 1+ \frac{1}{2\sqrt[3]{2}} + \frac{1}{3\sqrt[3]{3}} + \frac{1}{4\sqrt[3]{4}} + . . . }\) converges or diverges using the p-series convergence theorem.

Solution

255 video

video by PatrickJMT

Final Answer

The series \(\displaystyle{ 1+ \frac{1}{2\sqrt[3]{2}} + \frac{1}{3\sqrt[3]{3}} + \frac{1}{4\sqrt[3]{4}} + . . . }\) is a convergent p-series.

close solution

\(\displaystyle{ \sum_{n=1}^{\infty}{ \frac{1}{\sqrt[3]{n^4}} } }\)

Problem Statement

Determine whether the series \(\displaystyle{ \sum_{n=1}^{\infty}{ \frac{1}{\sqrt[3]{n^4}} } }\) converges or diverges using the p-series convergence theorem.

Solution

3158 video

close solution

\(\displaystyle{ \sum_{n=1}^{\infty}{ \sqrt[5]{n^2} } }\)

Problem Statement

Determine whether the series \(\displaystyle{ \sum_{n=1}^{\infty}{ \sqrt[5]{n^2} } }\) converges or diverges using the p-series convergence theorem.

Solution

3159 video

close solution

\(\displaystyle{ \sum_{n=1}^{\infty}{ \frac{3}{\sqrt[3]{n^2}} } }\)

Problem Statement

Determine whether the series \(\displaystyle{ \sum_{n=1}^{\infty}{ \frac{3}{\sqrt[3]{n^2}} } }\) converges or diverges using the p-series convergence theorem.

Solution

3160 video

close solution

\(\displaystyle{ \sum_{n=1}^{\infty}{ \frac{2}{n^3 \sqrt{n}} } }\)

Problem Statement

Determine whether the series \(\displaystyle{ \sum_{n=1}^{\infty}{ \frac{2}{n^3 \sqrt{n}} } }\) converges or diverges using the p-series convergence theorem.

Solution

3161 video

close solution

\(\displaystyle{ \sum_{n=1}^{\infty}{ \frac{4\sqrt{n}}{n^4} } }\)

Problem Statement

Determine whether the series \(\displaystyle{ \sum_{n=1}^{\infty}{ \frac{4\sqrt{n}}{n^4} } }\) converges or diverges using the p-series convergence theorem.

Solution

3162 video

close solution

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