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Subgroup of index two

Definition

Symbol-free definition

A subgroup of a group is said to be of index two if its index in the group is two, or equivalently, if it has exactly one coset other than itself.

Definition with symbols

A subgroup N of a group G is said to be of index two if [G:N] = 2.

Formalisms

First-order description

This subgroup property is a first-order subgroup property, viz., it has a first-order description in the theory of groups.
View a complete list of first-order subgroup properties

The property of being a subgroup of index two can be expressed in first-order logic (in fact, the property of being a subgroup of any fixed finite index can be expressed in first-order logic).

Relation with other properties

External links