Martens, Sebastian A.E. MPI for Comparative and International Private Law, Max Planck Society;
https://hwb-eup2009.mpipriv.de/index.php/Undue_influence (Any fulltext)
Martens, S. A. (2009). Undue influence. In J. Basedow, K. J. Hopt, & R. Zimmermann (Eds.), Handwörterbuch des Europäischen Privatrechts (pp. 1534-1537). Tübingen: Mohr Siebeck.