Blasi M.G, Lico R., Giroletti M., Orienti M., Giovannini G., Cotton W., Edwards P.G, Fuhrmann L., Krichbaum T.P, Kovalev Y.Y et al..  2013.  The TeV blazar Markarian 421 at the highest spatial resolution. Astronomy and Astrophysics. 559:A75.