15. Given the sets
$$
\begin{array}{l}
A=\left\{x \mid x=3^{n}, n \in \mathbf{N}_{+}\right\}, \\
B=\left\{x \mid x=4 n+1, n \in \mathbf{N}_{+}\right\} .
\end{array}
$$

Arrange the elements of $A \cap B$ in ascending order to form a sequence $\left\{a_{n}\right\}$. Then $a_{3}=$ $\qquad$ , and the general term formula for the sequence $\left\{a_{n}\right\}$ is $\qquad$
$$
15.729,9^{n}
$$

By observation and induction, it is easy to obtain.