thereby]{} a potential candidate [host for]{} a sub-parsec supermassive black hole binary.'
author:
- 'Yan-Rong Li'
- 'Jian-Min Wang'
- 'Zhi-Xiang Zhang'
- Kai Wang
- 'Ying-Ke Huang'
- 'Kai-Xing Lu'
- Chen Hu
- Pu Du
- Edi Bon
- 'Luis C. Ho'
- 'Jin-Ming Bai'
- 'Wei-Hao Bian'
- 'Ye-Fei Yuan'
- Hartmut Winkler
- 'Eduard K. Denissyuk'
- 'Rashit R. Valiullin'
- Nataša Bon
- 'Luka [Č.]{} Popovi[ć]{}'
title: |
    A Possible $\sim$20-Year Periodicity in Long-term Optical [Photometric and Spectral]{} Variations of the Nearby Radio-Quiet\
    Active Galactic Nucleus Ark 120
---

Introduction
============

[lccccccccc]{} D99b & 1 & 42,392-47,944 & 1974-1990 & 63 & $2.0\times7.0$ & 1.35 & $1.008\pm0.017$ & $\phm{+}2.05\pm0.25$\
P89 & 2 & 44,168-47,414 & 1979-1988 & 74 & $7.0$ & 0.925 & $1.005\pm0.021$ & $\phm{+}1.39\pm0.32$\
P98 & 3 & 47,524-50,388 & 1988-1996 & 141 & $5.0\times7.6$ & 0.91 & 1 & 0\
W96 & 4 & 48,619-48,834 & 1991-1992 & 20 & $5.0\times10.0$& 0.70 & $1.078\pm0.005$ & $\phm{+}0.46\pm0.06$\
D08 & 5 & 48,629-53,445 & 1992-2005 & 88 & $3.0\times11.0$& 1.13 & $1.037\pm0.003$ & $\phm{+}0.09\pm0.03$\
L & 6 & 57,295-57,839 & 2015-2017 & 100 & $2.5\times8.5$ & 0.96 & &\
M79 & 7 & 25,622-33,446 