\text { Use root or ratio test to determine the convergence of the series } \sum_{n=1}^{\infty} a_{n} \text { where } a_{n}=\left\{\begin{array}{ll}

\frac{1}{3^{k}} & \text { if } n=2 k \\

\frac{1}{9^{k}} & \text { if } n=2 k+1

\end{array}\right.