Double coset index of a subgroup

Symbol-free definition
The double coset index of a subgroup in a group is the cardinality of its double coset space, or equivalently, the number of double cosets the subgroup has.

Related subgroup properties

 * Subgroup of double coset index two

Facts

 * Double coset index two implies maximal
 * Bound on double coset index in terms of orders of group and subgroup
 * Index three implies normal or double coset index two
 * Index four implies 2-subnormal or double coset index two