How do you test for convergence of Sigma (-1)^n/sqrt(lnn) from n=[3,oo)?

1 Answer
Feb 3, 2017

The series converges by the alternating series test.

Explanation:

The alternating series test states that a series sum(-1)^na_n converges if |a_n| decreases monotonically and lim_(n->oo)a_n = 0.

In this case, we have an alternating series with a_n = 1/sqrt(ln(n)). As 0 < 1/sqrt(ln(n)) < 1/sqrt(ln(n+1)), we have that |a_n| is monotonically decreasing, and lim_(n->oo)a_n = lim_(n->oo)1/sqrt(ln(n)) = 1/oo = 0. Thus, by the alternating series test, the series converges.