 For example, if "R" is the ring of all square matrices of size "n" over a field, then formula_37 consists of the set of all invertible matrices of size "n", and is called the general linear group.A subset "S" of "R" is said to be a subring if it can be regarded as a ring with the addition and the multiplication restricted from "R" to "S".Equivalently, "S" is a subring if it is not empty, and for any "x", "y" in "S", formula_41, formula_42 and formula_43 are in "S".