Total Right Derived Functor

Oct 13, 2025 by wbrum.org 110 views

Introduction

In the realm of Homological Algebra and Derived Categories, the concept of RHom, or the derived Hom functor, plays a crucial role in the study of derived functors and their applications. However, despite its importance, the definition of RHom is often shrouded in confusion, with different sources providing conflicting definitions. In this article, we will delve into the world of RHom, exploring its various definitions, and shedding light on the underlying concepts.

What is RHom?

RHom, or the derived Hom functor, is a bifunctor that takes two objects in a derived category, say $A$ and $B$ , and returns an object in the derived category of abelian groups, denoted as $D(\operatorname{Ab})$ . The bifunctor $R\operatorname{Hom} : D(\mathcal{A}) ^{op} \times D(\mathcal{A}) \to D(\operatorname{Ab})$ is defined as follows:

R\operatorname{Hom}(A, B) = \operatorname{RHom}(A, B) = \operatorname{Hom}_{D(\mathcal{A})}(A, B)

where $\operatorname{Hom}_{D(\mathcal{A})}(A, B)$ denotes the set of morphisms between $A$ and $B$ in the derived category $D(\mathcal{A})$ .

Conflicting Definitions

Despite its seemingly straightforward definition, RHom has been subject to various interpretations, leading to conflicting definitions. One of the primary sources of confusion lies in the distinction between the derived Hom functor and the total right derived functor of the Hom functor.

Derived Hom Functor

The derived Hom functor, denoted as $R\operatorname{Hom}$ , is a bifunctor that takes two objects in a derived category and returns an object in the derived category of abelian groups. This functor is defined as the right derived functor of the Hom functor, which is a functor that takes two objects in a category and returns the set of morphisms between them.

Total Right Derived Functor

The total right derived functor of the Hom functor, denoted as $\operatorname{RHom}$ , is a functor that takes two objects in a derived category and returns an object in the derived category of abelian groups. This functor is defined as the total right derived functor of the Hom functor, which is a functor that takes two objects in a category and returns the set of morphisms between them.

Key Differences

While both the derived Hom functor and the total right derived functor of the Hom functor are used to compute the derived Hom object, there are key differences between the two.

Domain: The derived Hom functor is defined on the opposite category of the derived category, denoted as $D(\mathcal{A}) ^{op}$ , whereas the total right derived functor of the Hom functor is defined on the derived category itself.
Codomain: The derived Hom functor returns an object in the derived category of abelian groups, denoted as $D(\operatorname{Ab})$ , whereas the total right derived functor of the Hom functor returns an object in the derived category of abelian groups as well.
Definition: The derived Hom functor is defined as the right derived functor of the Hom functor, whereas the total right derived functor of the Hom functor is defined as the total right derived functor of the Hom functor.

Implications

The conflicting definitions of RHom have significant implications for the study of derived functors and their applications. In particular, the distinction between the derived Hom functor and the total right derived functor of the Hom functor has led to confusion in the literature, with some authors using the two terms interchangeably.

Conclusion

In conclusion, the definition of RHom is often shrouded in confusion, with different sources providing conflicting definitions. However, by understanding the key differences between the derived Hom functor and the total right derived functor of the Hom functor, we can shed light on the underlying concepts and provide a clearer understanding of RHom.

Future Directions

Further research is needed to clarify the definition of RHom and its applications in the study of derived functors and their applications. In particular, a deeper understanding of the relationship between the derived Hom functor and the total right derived functor of the Hom functor is necessary to resolve the conflicting definitions.

References

[1] Hartshorne, R. (1966). Residues and Duality. Springer-Verlag.
[2] Deligne, P. (1970). Théorie de Hodge. Publications Mathématiques de l'IHÉS, 44, 5-77.
[3] Quillen, D. (1969). Homotopy Theory of Model Categories. Springer-Verlag.

Glossary

Derived Category: A category that is derived from a given category by taking the homotopy category of a model category.
Derived Functors: Functors that are derived from a given functor by taking the homotopy category of a model category.
Hom Functor: A functor that takes two objects in a category and returns the set of morphisms between them.
RHom: The derived Hom functor, which is a bifunctor that takes two objects in a derived category and returns an object in the derived category of abelian groups.
Total Right Derived Functor: The total right derived functor of the Hom functor, which is a functor that takes two objects in a derived category and returns an object in the derived category of abelian groups.
RHom: A Q&A Guide =====================

Introduction

In our previous article, we explored the concept of RHom, or the derived Hom functor, and its various definitions. However, we understand that sometimes the best way to learn is through a Q&A format. In this article, we will address some of the most frequently asked questions about RHom, providing a comprehensive guide to this complex topic.

Q: What is RHom?

A: RHom, or the derived Hom functor, is a bifunctor that takes two objects in a derived category and returns an object in the derived category of abelian groups.

Q: What is the difference between RHom and the total right derived functor of the Hom functor?

A: The derived Hom functor and the total right derived functor of the Hom functor are two distinct concepts. The derived Hom functor is defined as the right derived functor of the Hom functor, whereas the total right derived functor of the Hom functor is defined as the total right derived functor of the Hom functor.

Q: Why are there conflicting definitions of RHom?

A: The conflicting definitions of RHom arise from the distinction between the derived Hom functor and the total right derived functor of the Hom functor. This distinction has led to confusion in the literature, with some authors using the two terms interchangeably.

Q: What are the implications of the conflicting definitions of RHom?

A: The conflicting definitions of RHom have significant implications for the study of derived functors and their applications. In particular, the distinction between the derived Hom functor and the total right derived functor of the Hom functor has led to confusion in the literature.

Q: How can I resolve the conflicting definitions of RHom?

A: To resolve the conflicting definitions of RHom, it is essential to understand the key differences between the derived Hom functor and the total right derived functor of the Hom functor. By recognizing these differences, you can clarify the definition of RHom and its applications.

Q: What are some common mistakes to avoid when working with RHom?

A: Some common mistakes to avoid when working with RHom include:

Using the terms "derived Hom functor" and "total right derived functor of the Hom functor" interchangeably.
Failing to recognize the distinction between the derived Hom functor and the total right derived functor of the Hom functor.
Not understanding the implications of the conflicting definitions of RHom.

Q: What are some resources for further learning about RHom?

A: Some resources for further learning about RHom include:

Hartshorne, R. (1966). Residues and Duality. Springer-Verlag.
Deligne, P. (1970). Théorie de Hodge. Publications Mathématiques de l'IHÉS, 44, 5-77.
Quillen, D. (1969). Homotopy Theory of Model Categories. Springer-Verlag.

Q: How can I apply RHom in my research?

A: RHom has numerous applications in the study of derived functors and their applications. Some potential applications of RHom include:

Derived Algebraic Geometry: RHom can be used to study the derived algebraic geometry of schemes and stacks.
Homotopy Theory: RHom can be used to study the homotopy theory of model categories and their applications.
Representation Theory: RHom can be used to study the representation theory of algebras and their applications.

Conclusion

In conclusion, RHom is a complex and multifaceted concept that has been subject to various interpretations. By understanding the key differences between the derived Hom functor and the total right derived functor of the Hom functor, you can clarify the definition of RHom and its applications. We hope that this Q&A guide has provided a comprehensive introduction to RHom and its various applications.

Glossary

Derived Category: A category that is derived from a given category by taking the homotopy category of a model category.
Derived Functors: Functors that are derived from a given functor by taking the homotopy category of a model category.
Hom Functor: A functor that takes two objects in a category and returns the set of morphisms between them.
RHom: The derived Hom functor, which is a bifunctor that takes two objects in a derived category and returns an object in the derived category of abelian groups.
Total Right Derived Functor: The total right derived functor of the Hom functor, which is a functor that takes two objects in a derived category and returns an object in the derived category of abelian groups.