The relentless variability of Mrk 421 from the TeV to the radio A. Arbet-Engels, D. Baack, M. Balbo, A. Biland, M. Blank, T. Bretz, K. Bruegge, M. Bulinski, J. Buss, M. Doerr et al. (15 more) A&A, 647 (2021) A88 Published online: 12 March 2021 DOI: 10.1051/0004-6361/201935557