1. Geometric sequence
$$
-a+\log _{3} 2017,2 a+\log _{2} 2017, -4 a+\log _{21} 2017
$$

The common ratio is $\qquad$ .
$$
-1 . \frac{8}{15} \text {. }
$$

Let these three terms be $x_{1}, x_{2}, x_{3}$, and the common ratio be $q$.
$$
\begin{array}{l}
\text { Then } q=\frac{x_{3}}{x_{2}}=\frac{x_{2}}{x_{1}}=\frac{x_{3}+2 x_{2}}{x_{2}+2 x_{1}} \\
=\frac{\log _{27} 2017+2 \log _{9} 2017}{\log _{9} 2017+2 \log _{3} 2017} \\
=\frac{\left(\frac{1}{3}+2 \times \frac{1}{2}\right) \log _{3} 2017}{\left(\frac{1}{2}+2\right) \log _{3} 2017}=\frac{8}{15} .
\end{array}
$$