This is a seemingly easy sequence but I cannot for the life of me figure it out:

Find a formula for the general term$\displaystyle a_n$ of the sequence, assuming the pattern given continues:

$\displaystyle a_n $= ?

{ $\displaystyle 1, \frac{1}{3}, \frac{1}{9}, \frac{1}{27}, \frac{1}{81}$ }