GRE Subject Test: Math : Comparing Rates of Convergence

Study concepts, example questions & explanations for GRE Subject Test: Math

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Example Questions

Example Question #121 : Convergence And Divergence

For which values of p is 

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

convergent?

Possible Answers:

\(\displaystyle p=2\)

All positive values of \(\displaystyle p\)

it doesn't converge for any values of \(\displaystyle p\)

only \(\displaystyle p > 1\)

Correct answer:

only \(\displaystyle p > 1\)

Explanation:

We can solve this problem quite simply with the integral test. We know that if 

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

converges, then our series converges. 

We can rewrite the integral as 

\(\displaystyle \int x^{-p} dx\)

and then use our formula for the antiderivative of power functions to get that the integral equals

\(\displaystyle \frac{x^{1-p}}{1-p} \Big|_{x=1}^\infty\).

We know that this only goes to zero if \(\displaystyle 1-p < 0\). Subtracting p from both sides, we get

\(\displaystyle 1 < p\).

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