Wloka, J. and Rowley, B. (1997) “New aspects of the $L$-condition for elliptic systems”, Topological Methods in Nonlinear Analysis, 10(2), pp. 283–294. Available at: https://apcz.umk.pl/TMNA/article/view/TMNA.1997.033 (Accessed: 3 July 2024).