The following paper is well known, but hard to find:
J. Boman, $L^p$-estimates for very strongly elliptic systems, Report 29, Department of Mathematics, University of Stockholm, 1982.
In this paper the so called Boman chain condition has been introduced. However, most of the people who quote this paper have never seen it (including myself). In most of the cases people follow other authors who quote the work of Boman.
Question. Does anyone have a pdf scan of the paper? Is it available anywhere in the internet?
Edit. Thanks to Dan Petersen who made the paper available, another question could be answered: https://mathoverflow.net/a/320747/121665