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The narrow class number of an algebraic number field is the order of its narrow class group. Since the ordinary ideal class group is a quotient of the narrow class group, the narrow class number is a multiple of the class number. Moreover, the ratio is a power of $2$. The two class numbers are the same in many cases, for example when the number field is totally complex.

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• Last edited by David Roberts on 2019-04-30 17:02:37
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