## 17Calculus Proofs - p-Series Proof

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This page contains a proof of the p-series convergence theorem. Using this theorem and practice problems are on a separate page.

p-Series Convergence Theorem

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

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.

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