Klaus, G. (1970). Was ist - was soll. Urania-Verlag.
Citazione stile Chigago Style (17a edizione)Klaus, Georg. Was Ist - Was Soll. Leipzig: Urania-Verlag, 1970.
Citatione MLA (8a ed.)Klaus, Georg. Was Ist - Was Soll. Urania-Verlag, 1970.
Attenzione: Queste citazioni potrebbero non essere precise al 100%.