Dissecting the long-term emission behaviour of the BL Lac object Mrk 421

NASA through Fermi Guest Investigator grants [NNX08AW56G, NNX09AU10G, NNX12AO93G, NNX15AU81G]; Shota Rustaveli NSF [FR/577/6-320/13]; US National Science Foundation [AST-1615796]; NASA Fermi Guest Investigator grant [NNX14AQ58G]; NASA through the Fermi Guest Investigator Program; NSF; BU; Lowell Observatory; Russian RFBR [15-02-00949]; St. Petersburg University [6.38.335.2015]; Ramon y Cajal grant of the Spanish Ministerio de Economia y Competitividad (MINECO); MINECO [AYA2010-14844, AYA2013-40825-P, AYA2016-80889-P]; Regional Government of Andalucia [P09-FQM-4784]; Scientific Research Fund of the Bulgarian Ministry of Education and Sciences [DO 02-137 (BIn-13/09), DN 08/1]; Institute of Astronomy; Rozhen National Astronomical Observatory; Bulgaria Academy of Sciences, via the bilateral joint research project 'Observations of ICRF radio-sources visible in optical domain'; Ministry of Education, Science and Technological Development of the Republic of Serbia [176011, 176004, 176021]; Shumen University [RD-08-81]