Give a Recursive Definition (With Initial Condition(s)) Of $$\left\{ a _ { n } \right\} ( n = 1,2,3 , \ldots )$$

give a recursive definition (with initial condition(s)) of $$\left\{ a _ { n } \right\} ( n = 1,2,3 , \ldots )$$
- $$a _ { n } = 2 ^ { n }$$