There are quite a few german mathematical theorems or notions which usually are not translated into other languages. For example,


[Nullstellensatz](http://en.wikipedia.org/wiki/Nullstellensatz), [Hauptvermutung](http://en.wikipedia.org/wiki/Hauptvermutung), [Freiheitssatz](http://en.wikipedia.org/wiki/Freiheitssatz), [Eigenvector](http://en.wikipedia.org/wiki/Eigenvector) (the "Eigen" part), [Verschiebung](http://arxiv.org/ftp/arxiv/papers/0804/0804.3888.pdf).


For me, as a German, this is quite entertaining. Do you know other examples? Please one per answer, please give a reference for the term or a short explanation of what it means.


It would be great to see an explanation why there is no translation.


EDIT: Some more examples can be found at [Wikipedia](http://en.wikipedia.org/wiki/List_of_German_expressions_in_English#Mathematics_and_formal_logic): Ansatz, Entscheidungsproblem, Grossencharakter, Hauptmodul, Möbius band, quadratfrei, Stützgerade, Vierergruppe, Nebentype.



German mathematical terms like "Nullstellensatz"
Führerdiskriminantenproduktformel. 


