2. Using the letters $a, b, c$ to form a word of length $n$ (i.e., a word composed of $n$ letters), if no two $a$s are adjacent, how many such words are there?
2. Let the number of words of length $n$ be $a_{n}$, then $a_{1}=3, a_{2}=8$, and $a_{n}=2 a_{n-1}+2 a_{n-2}(n \geqslant 3)$, solving this yields
$$
a_{n}=\frac{2+\sqrt{3}}{2 \sqrt{3}}(1+\sqrt{3})^{n}+\frac{-2+\sqrt{3}}{2 \sqrt{3}}(1-\sqrt{3})^{n} .
$$