Reichardt, W Forschungsgruppe Kybernetik, Max-Planck-Institut für Biologie, Max Planck Society;
https://link.springer.com/content/pdf/10.1007/BF02724690.pdf (Publisher version)
Reichardt, W. (1959). Analysis of a nonlinear biological filter. Nuovo Cimento: Series X, 13(Supplement), 608-616. doi:10.1007/BF02724690.