-21,8 °Ré reports Heinrich Wilhelm Matthias Olbers in a letter to Carl Friedrich Gauss from 6 February 1823, printed in: Carl Friedrich Gauß, Briefwechsel mit H.W.M. Olbers, Georg Olms Verlag, 1860 S. 233 (Bremen, p. 233, at Google Books).
Wolfgang Feist (2007-05-27). "Passivhaus-Schulgebäude" [Passive house school building] (in German). Passive House Institute. Archived from the original on 2007-08-27. Retrieved 2007-05-30.
"Bürgerschafts- und Beirätewahlen 2015, Vorläufiges Endergebnis" [2015 elections for Bürgerschaft and Beiräte (state, city, and local legislature), preliminary results] (in German). Statistisches Landesamt Bremen (Statistical Office of the State of Bremen). Archived from the original on 26 September 2017. Retrieved 18 May 2015.
"Bürgerschafts- und Beirätewahlen 2015, Vorläufiges Endergebnis" [2015 elections for Bürgerschaft and Beiräte (state, city, and local legislature), preliminary results] (in German). Statistisches Landesamt Bremen (Statistical Office of the State of Bremen). Archived from the original on 26 September 2017. Retrieved 18 May 2015.
Wolfgang Feist (2007-05-27). "Passivhaus-Schulgebäude" [Passive house school building] (in German). Passive House Institute. Archived from the original on 2007-08-27. Retrieved 2007-05-30.