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$\displaystyle \int \frac{1}{t} dt = \ln t$ diverges.

How do I show that $\displaystyle \int_2^\infty \frac{1}{t ~\log^2 t} dt$ is convergent?

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    On what interval?2012-08-03

1 Answers 1

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To help yourself figure out the appropriate substitution, reorganize the integral into: $ \int \frac{1}{\log^2 t} \frac{dt}{t} $ Does this ring a bell? $\dfrac{dt}{t}$? If we take $u = \log t,$ then we have $du = \dfrac{dt}{t}$ and $\dfrac{1}{\log^2 t} = \dfrac{1}{u^2}.$ So the integral is: $ \int \frac{1}{t\log^2 t} dt = \int \frac{1}{u^2} du = - \frac{1}{u} + \text{const} = -\frac{1}{\log t} + \text{const}.$ Now $ \int_2^{\infty} \frac{1}{t\log^2 t} dt = - \lim_{n \to \infty} \frac{1}{\log n} + \frac{1}{\log 2} = 0 + \frac{1}{\log 2}.$

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    Did you guess the integral's limits?2012-08-03