13. Let $s _ { n }$ be the sum of the first n terms of the geometric sequence $\left\{ a _ { n } \right\}$. If $a _ { 1 } = 1$ and $3 s _ { 1 } , 2 s _ { 2 } , s _ { 3 }$ form an arithmetic sequence, then $a _ { n } = $ $\_\_\_\_$.
13. Let $s _ { n }$ be the sum of the first n terms of the geometric sequence $\left\{ a _ { n } \right\}$. If $a _ { 1 } = 1$ and $3 s _ { 1 } , 2 s _ { 2 } , s _ { 3 }$ form an arithmetic sequence, then $a _ { n } = $ $\_\_\_\_$.