Siehr, Kurt MPI for Comparative and International Private Law, Max Planck Society;
https://max-eup2012.mpipriv.de/index.php/Groupe_Europ%C3%A9en_de_Droit_International_Priv%C3%A9 (Any fulltext)
Siehr, K. (2012). Groupe Européen de Droit International Privé. In J. Basedow, K. J. Hopt, & R. Zimmermann (Eds.), The Max Planck Encyclopedia of European Private Law (pp. 802-806). Oxford: Oxford University Press.