@Book{Klein-Barmen1956,
author  = "Klein-Barmen, Fritz",
title   = "Zur Axiomatik der Semigruppen",
year    = "1956",
address = "München",
series  = "Sitzungsberichte",
volume  = "1956,20",
url     = "https://publikationen.badw.de/de/003383886",
}