1. If $a+\log _{3} 2017, a+\log _{9} 2017, a+\log _{27} 2017(a \in R)$ form a geometric sequence, then its common ratio is $\qquad$
1. $\frac{1}{3}$.

Analysis: According to the problem, the common ratio $q=\frac{a+\log _{9} 2017}{a+\log _{3} 2017}=\frac{a+\log _{27} 2017}{a+\log _{9} 2017}=\frac{\log _{9} 2017-\log _{27} 2017}{\log _{3} 2017-\log _{9} 2017}$
$$
=\frac{\left(\frac{1}{2}-\frac{1}{3}\right) \log _{3} 2017}{\left(1-\frac{1}{2}\right) \log _{3} 2017}=\frac{1}{3}
$$