In the

mathematical Mathematics is an area of knowledge that includes the topics of numbers, formulas and related structures, shapes and the spaces in which they are contained, and quantities and their changes. These topics are represented in modern mathematics ...

field of

group theory In abstract algebra, group theory studies the algebraic structures known as group (mathematics), groups. The concept of a group is central to abstract algebra: other well-known algebraic structures, such as ring (mathematics), rings, field ...

, a group is residually ''X'' (where ''X'' is some property of groups) if it "can be recovered from groups with property ''X''". Formally, a group ''G'' is residually ''X'' if for every non-trivial element ''g'' there is a

homomorphism In algebra, a homomorphism is a structure-preserving map between two algebraic structures of the same type (such as two groups, two rings, or two vector spaces). The word ''homomorphism'' comes from the Ancient Greek language: () meaning "same" ...

''h'' from ''G'' to a group with property ''X'' such that

h(g)\neq e

. More categorically, a group is residually ''X'' if it embeds into its pro-''X'' completion (see profinite group, pro-p group), that is, the

inverse limit In mathematics, the inverse limit (also called the projective limit) is a construction that allows one to "glue together" several related objects, the precise gluing process being specified by morphisms between the objects. Thus, inverse limits can ...

of the

inverse system In mathematics, the inverse limit (also called the projective limit) is a construction that allows one to "glue together" several related objects, the precise gluing process being specified by morphisms between the objects. Thus, inverse limits ca ...

consisting of all morphisms

\phi\colon G \to H

from ''G'' to some group ''H'' with property ''X''.

Examples

Important examples include: *

Residually finite {{unsourced, date=September 2022 In the mathematical field of group theory, a group ''G'' is residually finite or finitely approximable if for every element ''g'' that is not the identity in ''G'' there is a homomorphism ''h'' from ''G'' to a fini ...

* Residually

nilpotent In mathematics, an element x of a ring R is called nilpotent if there exists some positive integer n, called the index (or sometimes the degree), such that x^n=0. The term was introduced by Benjamin Peirce in the context of his work on the cla ...

* Residually solvable * Residually free

References

* Infinite group theory Properties of groups {{Abstract-algebra-stub