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From: William Elliot on 13 Aug 2010 06:52
If I is an ideal of a ring R and [R:I] = 2, then I is a maximal ideal.

Are there any other benifits from an ideal having an index of two?

Are there ideals with an index of two that aren't prime?

From: Hagen on 13 Aug 2010 04:34
> Are there ideals with an index of two that aren't
> prime?


Yes. Take any abelian group R with a subgroup I of
index 2. Define multiplication through a*b = 0 for
all a,b \in G.

H
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