6 Subgroups

See also Subgroup.

It’s not clear if the author’s definition of “regular” is anything but a completely custom (and informal) description specific to the author. The author does provide a reference to a more technical definition in the footnote on pg. 98, however.

6.4 Cosets

See also Coset.

6.4 Lagrange’s theorem

See also Lagrange’s theorem (group theory).