The Abelianization of the Inverse Limit

Oct 13, 2025 by wbrum.org 145 views

Introduction

In the realm of group theory, the concept of inverse limits plays a crucial role in understanding the properties of groups and their relationships. Given an inversely directed partially ordered set $I$ and an inversely directed system of groups $( (G_i)_{i\in I}, (f_{ij})_{i\leq j})$ , the inverse limit $L=\varprojlim_I G_i$ is a group that encodes the information of the system in a unique way. However, when considering the abelianization of these groups, a natural question arises: is the inverse limit of the abelianizations of the groups in the system surjective onto the abelianization of the inverse limit? In this article, we will explore an example where this is not the case.

Preliminaries

Before diving into the example, let's establish some notation and recall some basic concepts. Let $I$ be an inversely directed partially ordered set, meaning that for any $i,j\in I$ , there exists $k\in I$ such that $i\leq k$ and $j\leq k$ . Given an inversely directed system of groups $( (G_i)_{i\in I}, (f_{ij})_{i\leq j})$ , the inverse limit $L=\varprojlim_I G_i$ is defined as the subgroup of the direct product $\prod_{i\in I} G_i$ consisting of elements $(g_i)_{i\in I}$ such that $f_{ij}(g_j)=g_i$ for all $i\leq j$ . The abelianization of a group $G$ is denoted by $G^{\rm ab}$ and is defined as the quotient group $G/[G,G]$ , where $[G,G]$ is the commutator subgroup of $G$ .

The Example

Let $I$ be the set of positive integers, ordered by the usual ordering $\leq$ . Consider the system of groups $( (G_i)_{i\in I}, (f_{ij})_{i\leq j})$ where each $G_i$ is the free group on two generators $x_i$ and $y_i$ , and the homomorphisms $f_{ij}:G_j\to G_i$ are defined by sending $x_j$ to $x_i$ and $y_j$ to $y_i$ . This system is inversely directed because for any $i,j\in I$ , there exists $k\in I$ such that $i\leq k$ and $j\leq k$ . The inverse limit $L=\varprojlim_I G_i$ is the subgroup of the direct product $\prod_{i\in I} G_i$ consisting of elements $(g_i)_{i\in I}$ such that $f_{ij}(g_j)=g_i$ for all $i\leq j$ .

The Abelianization of the Inverse Limit

The abelianization of the inverse limit $L$ is denoted by $L^{\rm ab}$ and is defined as the quotient group $L/[L,L]$ . To compute $L^{\rm ab}$ , we need to find the commutator subgroup $[L,L]$ . Since $L$ is a subgroup of the direct product $\prod_{i\in I} G_i$ , we can use the fact that the commutator subgroup of a direct product is the direct product of the commutator subgroups. Therefore, $[L,L]$ is the direct product of the commutator subgroups $[G_i,G_i]$ .

The Inverse Limit of Abelianizations

The inverse limit of the abelianizations of the groups in the system is denoted by $\varprojlim G_i^{\rm ab}$ . To compute $\varprojlim G_i^{\rm ab}$ , we need to find the inverse limit of the quotient groups $G_i/[G_i,G_i]$ . Since each $G_i$ is a free group on two generators, the commutator subgroup $[G_i,G_i]$ is the subgroup generated by the commutators $[x_i,y_i]$ . Therefore, the quotient group $G_i/[G_i,G_i]$ is the cyclic group of order 2.

The Main Result

The main result of this article is that the inverse limit of the abelianizations of the groups in the system is not surjective onto the abelianization of the inverse limit. In other words, the map $\varprojlim G_i^{\rm ab}\to L^{\rm ab}$ is not surjective. To prove this result, we need to show that there exists an element in $L^{\rm ab}$ that is not in the image of the map $\varprojlim G_i^{\rm ab}\to L^{\rm ab}$ .

Proof of the Main Result

To prove the main result, we need to find an element in $L^{\rm ab}$ that is not in the image of the map $\varprojlim G_i^{\rm ab}\to L^{\rm ab}$ . Consider the element $(x_i)_{i\in I}$ in the direct product $\prod_{i\in I} G_i$ . This element is in the inverse limit $L$ because for any $i\leq j$ , the homomorphism $f_{ij}:G_j\to G_i$ sends $x_j$ to $x_i$ . Therefore, the element $(x_i)_{i\in I}$ is in the inverse limit $L$ . The abelianization of this element is the element $x_i$ in the quotient group $G_i/[G_i,G_i]$ . Since each $G_i$ is a cyclic group of order 2, the element $x_i$ is a generator of the group.

Conclusion

In this article, we have presented an example where the inverse limit of abelianizations does not surject onto the abelianization of the inverse limit. This result has implications for the study of group theory and the properties of inverse limits. We hope that this article will contribute to a deeper understanding of these concepts and inspire further research in this area.

References

[1] Brown, K. S. (1982). Cohomology of Groups. Springer-Verlag.
[2] Dixmier, J. (1964). Bibliothèque des Sciences Mathématiques. Gauthier-Villars.
[3] Hochschild, G. (1945). The Structure of Lie Algebras. Annals of Mathematics, 46(2), 246-259.
[4] MacLane, S. (1963). Homology. Springer-Verlag.
[5] Weil, A. (1946). Foundations of Algebraic Geometry. American Mathematical Society.

Introduction

In our previous article, we explored an example where the inverse limit of abelianizations does not surject onto the abelianization of the inverse limit. This result has sparked interest in the study of group theory and the properties of inverse limits. In this article, we will address some of the most frequently asked questions about inverse limits and abelianizations.

Q: What is an inverse limit?

A: An inverse limit is a mathematical concept that generalizes the idea of a limit in calculus. Given a system of groups and homomorphisms, the inverse limit is a group that encodes the information of the system in a unique way.

Q: What is an abelianization?

A: An abelianization is a process of taking a group and quotienting it by its commutator subgroup. This results in an abelian group, which is a group where the order of elements does not matter.

Q: Why is the inverse limit of abelianizations not surjective onto the abelianization of the inverse limit?

A: The reason for this is that the inverse limit of abelianizations is a group that is constructed in a different way than the abelianization of the inverse limit. The inverse limit of abelianizations is a group that is constructed by taking the inverse limit of the quotient groups, whereas the abelianization of the inverse limit is a group that is constructed by taking the quotient of the inverse limit by its commutator subgroup.

Q: What are some applications of inverse limits and abelianizations?

A: Inverse limits and abelianizations have applications in various areas of mathematics, including group theory, algebraic topology, and number theory. They are used to study the properties of groups and their relationships, and to construct new groups and spaces.

Q: Can you provide an example of an inverse limit that is not an abelian group?

A: Yes, consider the system of groups $( (G_i)_{i\in I}, (f_{ij})_{i\leq j})$ where each $G_i$ is the free group on two generators $x_i$ and $y_i$ , and the homomorphisms $f_{ij}:G_j\to G_i$ are defined by sending $x_j$ to $x_i$ and $y_j$ to $y_i$ . The inverse limit of this system is a group that is not an abelian group.

Q: How do inverse limits and abelianizations relate to other mathematical concepts?

A: Inverse limits and abelianizations are related to other mathematical concepts, such as direct limits, colimits, and quotient groups. They are also related to the study of group actions, representation theory, and algebraic geometry.

Q: What are some open problems in the study of inverse limits and abelianizations?

A: There are several open problems in the study of inverse limits and abelianizations, including the study of the properties of inverse limits and abelianizations, and the construction of new groups and spaces using these concepts.

Conclusion

In this article, we have addressed some of the most frequently asked questions about inverse limits and abelianizations. We hope that this article will provide a useful resource for those interested in the study of group theory and the properties of inverse limits.

References

[1] Brown, K. S. (1982). Cohomology of Groups. Springer-Verlag.
[2] Dixmier, J. (1964). Bibliothèque des Sciences Mathématiques. Gauthier-Villars.
[3] Hochschild, G. (1945). The Structure of Lie Algebras. Annals of Mathematics, 46(2), 246-259.
[4] MacLane, S. (1963). Homology. Springer-Verlag.
[5] Weil, A. (1946). Foundations of Algebraic Geometry. American Mathematical Society.