Show divergence of a real series. [closed]

Let $a_n = \int_0^1 (1 - x ^ 2) ^ n dx$ for $ n\ge 1$.Show that $ \sum_{n=1} ^\infty a_n $ diverges.

Here we can show that {$a_n$} converges to zero by using monotonic convergence theorem as $a_n$ is a decreasing sequence having lower bound zero.But I'm not getting how to proceed further to prove divergence of the series. Cauchy root test and ratio test is becoming too complicated. Can someone help me to solve this?