In the mathematical subject of group theory, a co-Hopfian group is a group that is not isomorphic to any of its proper subgroups. The notion is dual to that of a Hopfian group, named after Heinz Hopf. ^[1]

Formal definition

A group G is called co-Hopfian if whenever ${\displaystyle \varphi :G\to G}$ is an injective group homomorphism then ${\displaystyle \varphi }$ is surjective, that is ${\displaystyle \varphi (G)=G}$.^[2]

Examples and non-examples

• Every finite group G is co-Hopfian.
• The infinite cyclic group ${\displaystyle \mathbb {Z} }$ is not co-Hopfian since ${\displaystyle f:\mathbb {Z} \to \mathbb {Z} ,f(n)=2n}$ is an injective but non-surjective homomorphism.
• The additive group of real numbers ${\displaystyle \mathbb {R} }$ is not co-Hopfian, since ${\displaystyle \mathbb {R} }$ is an infinite-dimensional vector space over ${\displaystyle \mathbb {Q} }$ and therefore, as a group ${\displaystyle \mathbb {R} \cong \mathbb {R} \times \mathbb {R} }$.^[2]
• The additive group of rational numbers ${\displaystyle \mathbb {Q} }$ and the quotient group ${\displaystyle \mathbb {Q} /\mathbb {Z} }$ are co-Hopfian.^[2]
• The multiplicative group ${\displaystyle \mathbb {Q} ^{\ast }}$ of nonzero rational numbers is not co-Hopfian, since the map ${\displaystyle \mathbb {Q} ^{\ast }\to \mathbb {Q} ^{\ast },q\mapsto \operatorname {sign} (q)\,q^{2}}$ is an injective but non-surjective homomorphism.^[2] In the same way, the group ${\displaystyle \mathbb {Q} _{+}^{\ast }}$ of positive rational numbers is not co-Hopfian.
• The multiplicative group ${\displaystyle \mathbb {C} ^{\ast }}$ of nonzero complex numbers is not co-Hopfian.^[2]
• For every ${\displaystyle n\geq 1}$ the free abelian group ${\displaystyle \mathbb {Z} ^{n}}$ is not co-Hopfian.^[2]
• For every ${\displaystyle n\geq 1}$ the free group ${\displaystyle F_{n}}$ is not co-Hopfian.^[2]
• There exists a finitely generated non-elementary (that is, not virtually cyclic) virtually free group which is co-Hopfian. Thus a subgroup of finite index in a finitely generated co-Hopfian group need not be co-Hopfian, and being co-Hopfian is not a quasi-isometry invariant for finitely generated groups.^[3]
• Baumslag–Solitar groups ${\displaystyle BS(1,m)}$, where ${\displaystyle m\geq 1}$, are not co-Hopfian.^[4]
• If G is the fundamental group of a closed aspherical manifold with nonzero Euler characteristic (or with nonzero simplicial volume or nonzero L²-Betti number), then G is co-Hopfian.^[5]
• If G is the fundamental group of a closed connected oriented irreducible 3-manifold M then G is co-Hopfian if and only if no finite cover of M is a torus bundle over the circle or the product of a circle and a closed surface.^[6]
• If G is an irreducible lattice in a real semi-simple Lie group and G is not a virtually free group then G is co-Hopfian.^[7] E.g. this fact applies to the group ${\displaystyle SL(n,\mathbb {Z} )}$ for ${\displaystyle n\geq 3}$.
• If G is a one-ended torsion-free word-hyperbolic group then G is co-Hopfian, by a result of Sela.^[8]
• If G is the fundamental group of a complete finite volume smooth Riemannian n-manifold (where n > 2) of pinched negative curvature then G is co-Hopfian. ^[9]
• The mapping class group of a closed hyperbolic surface is co-Hopfian.^[10]
• The group Out(F_n) (where n>2) is co-Hopfian.^[11]
• Delzant and Polyagailo gave a characterization of co-Hopficity for geometrically finite Kleinian groups of isometries of ${\displaystyle \mathbb {H} ^{n}}$ without 2-torsion.^[12]
• A right-angled Artin group ${\displaystyle A(\Gamma )}$ (where ${\displaystyle \Gamma }$ is a finite nonempty graph) is not co-Hopfian; sending every standard generator of ${\displaystyle A(\Gamma )}$ to a power ${\displaystyle >1}$ defines and endomorphism of ${\displaystyle A(\Gamma )}$ which is injective but not surjective.^[13]
• A finitely generated torsion-free nilpotent group G may be either co-Hopfian or not co-Hopfian, depending on the properties of its associated rational Lie algebra.^[5][3]
• If G is a relatively hyperbolic group and ${\displaystyle \varphi :G\to G}$ is an injective but non-surjective endomorphism of G then either ${\displaystyle \varphi ^{k}(G)}$ is parabolic for some k >1 or G splits over a virtually cyclic or a parabolic subgroup.^[14]
• Grigorchuk group G of intermediate growth is not co-Hopfian.^[15]
• Thompson group F is not co-Hopfian.^[16]
• There exists a finitely generated group G which is not co-Hopfian but has Kazhdan's property (T).^[17]
• If G is Higman's universal finitely presented group then G is not co-Hopfian, and G cannot be embedded in a finitely generated recursively presented co-Hopfian group.^[18]

Generalizations and related notions

• A group G is called finitely co-Hopfian^[19] if whenever ${\displaystyle \varphi :G\to G}$ is an injective endomorphism whose image has finite index in G then ${\displaystyle \varphi (G)=G}$. For example, for ${\displaystyle n\geq 2}$ the free group ${\displaystyle F_{n}}$ is not co-Hopfian but it is finitely co-Hopfian.
• A finitely generated group G is called scale-invariant if there exists a nested sequence of subgroups of finite index of G, each isomorphic to G, and whose intersection is a finite group.^[4]
• A group G is called dis-cohopfian^[3] if there exists an injective endomorphism ${\displaystyle \varphi :G\to G}$ such that ${\displaystyle \bigcap _{n=1}^{\infty }\varphi ^{n}(G)=\{1\}}$.
• In coarse geometry, a metric space X is called quasi-isometrically co-Hopf if every quasi-isometric embedding ${\displaystyle f:X\to X}$ is coarsely surjective (that is, is a quasi-isometry). Similarly, X is called coarsely co-Hopf if every coarse embedding ${\displaystyle f:X\to X}$ is coarsely surjective. ^[20]
• In metric geometry, a metric space K is called quasisymmetrically co-Hopf if every quasisymmetric embedding ${\displaystyle K\to K}$ is onto. ^[21]

See also

• Hopfian object

References

Further reading

• K. Varadarajan, Hopfian and co-Hopfian Objects, Publicacions Matemàtiques 36 (1992), no. 1, pp. 293–317