@Book{Liebmann1918,
author  = "Liebmann, Heinrich",
title   = "Integralinvarianten und isoperimetrische Probleme",
year    = "1918",
address = "München",
series  = "Sitzungsberichte",
volume  = "1918,20",
url     = "https://publikationen.badw.de/en/003399194",
}