1. If $a+\log _{3} 2016, a+\log _{9} 2016, a+\log _{27} 2016(a \in \mathbf{R})$ form a geometric sequence, then its common ratio is
$\qquad$ (Yang Yunxin provided the problem)
1. $\frac{1}{3}$.

Let the required common ratio be $q$, then
$$
\begin{aligned}
q & =\frac{a+\log _{27} 2016}{a+\log _{9} 2016}=\frac{a+\log _{9} 2016}{a+\log _{3} 2016}=\frac{\log _{27} 2016-\log _{9} 2016}{\log _{9} 2016-\log _{3} 2016} \\
& =\frac{\frac{1}{3} t-\frac{1}{2} t}{\frac{1}{2} t-t}=\frac{1}{3}
\end{aligned}
$$

Here, $t=\log _{3} 2016$.