U21 (オーストリア＝ハンガリー帝国潜水艦)
Gardiner, Robert, ed. (1985). Conway's All the World's Fighting Ships, 1906–1921. Annapolis, Maryland: Naval Institute Press. ISBN 9780870219078. OCLC 12119866
Gibson, R. H.; Maurice Prendergast (2003) [1931]. The German Submarine War, 1914–1918. Annapolis, Maryland: Naval Institute Press. ISBN 9781591143147. OCLC 52924732
Halpern, Paul G. (1994). A Naval History of World War I. Annapolis, Maryland: Naval Institute Press. ISBN 9780870212666. OCLC 28411665
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