Convergence of the power series

47 Views Asked by At

Does the power series $\sum_{n=1}(1 - 1/sqrt(n))^n $ converge or diverge?

I tried root test but it doesn't work. Which convergence test should I try to solve the problem?

1

There are 1 best solutions below

7
On BEST ANSWER

Outline: After a while, $\left(1-\frac{1}{\sqrt{n}}\right)^{\sqrt{n}}$ will be close to $e^{-1}$, and in particular will be positive and less than $\frac{1}{2}$.

Thus after a while $\left(1-\frac{1}{\sqrt{n}}\right)^{n}$ is positive and less than $\frac{1}{2^{\sqrt{n}}}$.

To finish, show that after a while $2^{\sqrt{n}}\gt n^2$. From this we can conclude that our series converges.