 Solution

ID: past-exam-of-the-mathematics-course-of-the-year-2-of-the-natural-sciences-course-of-the-university-of-cambridge/2017/paper-1/7/c/i/solution

a_{n} = 1/ ln n

R^{- 1} = lim sup ∣ a_{n} ∣^{1/ n}

. Here

∣ a_{n} ∣^{1/ n} = exp [- \frac{l n l n n}{n}] ⟶ 1.

Therefore

R = 1.

Omitting the first two powers has no effect on the radius of convergence.

 New to topics? Read the docs here!