Martens, Sebastian A.E. MPI for Comparative and International Private Law, Max Planck Society;
https://max-eup2012.mpipriv.de/index.php/Undue_Influence (Any fulltext)
Martens, S. A. (2012). Undue Influence. In J. Basedow, K. J. Hopt, & R. Zimmermann (Eds.), The Max Planck Encyclopedia of European Private Law (pp. 1708-1712). Oxford: Oxford University Press.