@electronic{www.mathekalender.de,
   title       = {Digitaler Adventskalender - Kalender}, 
   url         = {https://www.mathekalender.de/index.php?page=calendar},
   biburl      = {https://puma.uni-kassel.de/url/76a89eedfa3cbbf4d7f2deaf1f07a764/stephandoerfel},
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@electronic{www.mathe-spass.de,
   title       = {Knobeln Mathematischer Unfug}, 
   url         = {http://www.mathe-spass.de/error.htm},
   biburl      = {https://puma.uni-kassel.de/url/452693ebea7a7c040f014114c8be7551/stephandoerfel},
   keywords    = {beweis fehler knobeln mathe spass},
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