# Why is the identity element of a group denoted by $e$?

The question was asked by a student, and I did not have a ready answer. I can think of the German word Einheit'', but since in German that is not how the identity element of a group is called, I doubt that is the origin. Any ideas?

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"but since in German that is not how the identity element of a group is called" ... Sometimes it is indeed called like this. Also the identity matrix is frequently or at least not rarely called 'Einheitsmatrix'. Another thought: Sometimes the identity element in a multiplicative group is called (perhaps sloppily) Einselement (where 'eins' means 'one'). –  quid Feb 7 '12 at 14:09

Heinrich Weber uses Einheit and e in his Lehrbuch der Algebra (1896).

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That is almost certainly the origin, though it should be noted that one in Russian is "edinica". –  Igor Rivin Feb 7 '12 at 14:58
@Igor: The influential early textbooks on algebra tended to be written in German, unfair though that may be to those of us who grew up with English (or Russian). Quite a bit of common terminology and notation in mathematics seems to have originated in German work during the 19th century, such as the symbols $K,k$ for fields. –  Jim Humphreys Feb 7 '12 at 20:55
Well, Weber surely popularized the term. But his friend Dedekind used "einheit" before him to mean either a unit in a field, or a unit measure in geometry, and I'll bet if you look in his work you'll find it for groups. Probably if you dig into the 19th century you can find a series of earlier and earlier, vaguer and vaguer, uses of the term for a group identity. –  Colin McLarty Dec 29 '12 at 17:35
In todays German literature, it seems that Einselement (or even neutrales Element) is preferred over Einheit (which is used for units, i.e. invertible elements of a ring). –  Hagen von Eitzen Feb 8 '14 at 21:49

The identity element for a complex number is (1,0)=1. The german word for "one" is "eins" so we write e.

http://de.wikipedia.org/wiki/Neutrales_Element

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