4 \text { a.) } \sum_{n=2}^{\infty} \frac{\ln n}{n^{2}} \text { b.) } \sum_{n=1}^{\infty} \ln \left(\frac{n^{2}+1}{2 n^{2}+1}\right) \text { c.) } \sum_{n=1}^{\infty} \frac{1}{(2 n+1)^{3}} \text { d.) } \sum_{n=1}^{\infty}\left(\frac{n^{2}+1}{2 n^{2}+1}\right)^{n}