@Book{Wilkens1956,
author  = "Wilkens, Alexander",
title   = "Über die Integral-Invarianten der Störungstheorie",
year    = "1956",
address = "München",
series  = "Sitzungsberichte",
volume  = "1955,7",
url     = "https://publikationen.badw.de/en/003383828",
}