by M. Marvan
Acta Appl. Math. 54 (1998) 1–25
Following Krasil'shchik and Vinogradov, Bäcklund transformations are defined as a pair of coverings with a common total space. We consider two local properties inspired by this picture. Normality implies infinite generating power. Effectivity is a precondition to normality. We check normality for the pKdV, sine-Gordon, and Tzitzeica equation. One naturally conjectures that completely integrable equations have normal Bäcklund transformations.