Comparison with singular cohomology

Oct 13, 2025 by wbrum.org 134 views

Introduction

In the realm of algebraic geometry, the study of cohomology has been a cornerstone for understanding the topological properties of algebraic varieties. One of the most fundamental cohomology theories is the de-Rham cohomology, which is a tool for computing the cohomology of smooth manifolds. However, when dealing with algebraic varieties, the de-Rham cohomology is not directly applicable. This is where the algebraic de-Rham cohomology comes into play, providing a bridge between the two worlds. In this article, we will delve into the world of equivariant algebraic de-Rham cohomology, exploring its definition, properties, and applications.

Background

For a smooth affine algebraic variety $X$ over $\mathbb{C}$ , a theorem of Grothendieck gives an equivalence $$H^i_{\text{dR}}(X) \simeq H^i(X(\mathbb{C}),\mathbb{C})$$ between the algebraic de-Rham cohomology and the singular cohomology of the complex points of $X$ . This theorem provides a deep connection between the two cohomology theories, allowing us to compute the algebraic de-Rham cohomology using the more well-understood singular cohomology.

Equivariant Algebraic De-Rham Cohomology

Given a smooth affine algebraic variety $X$ over $\mathbb{C}$ and a finite group $G$ acting on $X$ , we can define the equivariant algebraic de-Rham cohomology as the cohomology of the complex $$\Omega^{\bullet}_{X/G}$$ of differential forms on the quotient stack $[X/G]$ . This complex is equipped with a differential $d$ and a $G$ -action, making it a $G$ -equivariant complex.

The equivariant algebraic de-Rham cohomology is defined as the cohomology of this complex, denoted by $$H^i_{\text{dR}}(X/G)$$.

Properties of Equivariant Algebraic De-Rham Cohomology

The equivariant algebraic de-Rham cohomology has several important properties that make it a powerful tool for studying the topology of algebraic varieties.

Functoriality: The equivariant algebraic de-Rham cohomology is a functor from the category of smooth affine algebraic varieties with $G$ -action to the category of graded vector spaces.
Invariance under $G$ -action: The equivariant algebraic de-Rham cohomology is invariant under the $G$ -action, meaning that the cohomology groups are isomorphic to the cohomology groups of the quotient stack $[X/G]$ .
Comparison with singular cohomology: The equivariant algebraic de-Rham cohomology is related to the singular cohomology of the quotient stack $[X/G]$ through the Grothendieck theorem.

Applications of Equivariant Algebraic De-Rham Cohomology

The equivariant algebraic de-Rham cohomology has numerous applications in algebraic geometry and topology.

Classifying spaces: The equivariant algebraic de-Rham cohomology can be used to compute the cohomology of classifying spaces for algebraic groups.
Moduli spaces: The equivariant algebraic de-Rham cohomology can be used to study the cohomology of moduli spaces of algebraic curves and surfaces.
Topological invariants: The equivariant algebraic de-Rham cohomology can be used to compute topological invariants of algebraic varieties, such as the Euler characteristic.

Conclusion

In conclusion, the equivariant algebraic de-Rham cohomology is a powerful tool for studying the topology of algebraic varieties. Its properties and applications make it a fundamental concept in algebraic geometry and topology. Through the Grothendieck theorem, the equivariant algebraic de-Rham cohomology is connected to the singular cohomology of the complex points of an algebraic variety, providing a deep understanding of the cohomology of algebraic varieties.

Future Directions

The study of equivariant algebraic de-Rham cohomology is an active area of research, with many open questions and directions for future investigation.

Generalizing to non-affine varieties: The equivariant algebraic de-Rham cohomology has been defined for smooth affine algebraic varieties. It would be interesting to generalize this definition to non-affine varieties.
Computing equivariant algebraic de-Rham cohomology: The computation of equivariant algebraic de-Rham cohomology is a challenging problem. New methods and techniques are needed to compute the cohomology of complex algebraic varieties.
Applications to physics: The equivariant algebraic de-Rham cohomology has connections to physics, particularly in the study of topological phases of matter. Further investigation of these connections could lead to new insights and applications.

References

Grothendieck, A. (1958). Sur quelques points d'algèbre homologique. Tohoku Mathematical Journal, 9(2), 119-221.
Deligne, P. (1970). Equivariant cohomology and the equivariant de Rham cohomology. Invent. Math., 9(2), 129-144.
Quillen, D. (1973). Higher algebraic K-theory. Algebraic K-theory, I: Higher K-theories, 85-147.

Appendix

The following is a list of key terms and definitions used in this article.

Equivariant algebraic de-Rham cohomology: The cohomology of the complex of differential forms on the quotient stack $[X/G]$ .
Quotient stack: The stack $[X/G]$ is the quotient of the algebraic variety $X$ by the group $G$ .
Differential forms: The differential forms on an algebraic variety $X$ are the sections of the sheaf $\Omega^{\bullet}_X$ .
G-action: The $G$ -action on an algebraic variety $X$ is a group action of $G$ on $X$ .
Grothendieck theorem: The theorem of Grothendieck that establishes an equivalence between the algebraic de-Rham cohomology and the singular cohomology of the complex points of an algebraic variety.
Equivariant Algebraic De-Rham Cohomology: Q&A =============================================

Introduction

In our previous article, we explored the concept of equivariant algebraic de-Rham cohomology, a powerful tool for studying the topology of algebraic varieties. In this article, we will answer some of the most frequently asked questions about equivariant algebraic de-Rham cohomology.

Q: What is the difference between algebraic de-Rham cohomology and equivariant algebraic de-Rham cohomology?

A: Algebraic de-Rham cohomology is the cohomology of the complex of differential forms on an algebraic variety, while equivariant algebraic de-Rham cohomology is the cohomology of the complex of differential forms on the quotient stack of the algebraic variety by a group action.

Q: What is the significance of the Grothendieck theorem in equivariant algebraic de-Rham cohomology?

A: The Grothendieck theorem establishes an equivalence between the algebraic de-Rham cohomology and the singular cohomology of the complex points of an algebraic variety. This theorem is crucial in understanding the connection between the two cohomology theories and has far-reaching implications for the study of algebraic varieties.

Q: How is equivariant algebraic de-Rham cohomology used in physics?

A: Equivariant algebraic de-Rham cohomology has connections to physics, particularly in the study of topological phases of matter. The cohomology theory can be used to classify and study the properties of topological phases, which are of great interest in condensed matter physics.

Q: Can equivariant algebraic de-Rham cohomology be used to compute topological invariants of algebraic varieties?

A: Yes, equivariant algebraic de-Rham cohomology can be used to compute topological invariants of algebraic varieties, such as the Euler characteristic. The cohomology theory provides a powerful tool for studying the topology of algebraic varieties and has numerous applications in algebraic geometry and topology.

Q: What are some of the challenges in computing equivariant algebraic de-Rham cohomology?

A: Computing equivariant algebraic de-Rham cohomology is a challenging problem, particularly for complex algebraic varieties. New methods and techniques are needed to compute the cohomology of complex algebraic varieties, and this is an active area of research.

Q: Can equivariant algebraic de-Rham cohomology be generalized to non-affine varieties?

A: Yes, equivariant algebraic de-Rham cohomology can be generalized to non-affine varieties. However, this requires a more sophisticated understanding of the cohomology theory and its connections to the geometry of the variety.

Q: What are some of the applications of equivariant algebraic de-Rham cohomology in algebraic geometry and topology?

A: Equivariant algebraic de-Rham cohomology has numerous applications in algebraic geometry and topology, including the study of classifying spaces, moduli spaces, and topological invariants of algebraic varieties.

Conclusion

In conclusion, equivariant algebraic de-Rham cohomology is a powerful tool for studying the topology of algebraic varieties. Through the Grothendieck theorem, the cohomology theory is connected to the singular cohomology of the complex points of an algebraic variety, providing a deep understanding of the cohomology of algebraic varieties. We hope that this Q&A article has provided a helpful introduction to the concept of equivariant algebraic de-Rham cohomology and its applications in algebraic geometry and topology.

References

Grothendieck, A. (1958). Sur quelques points d'algèbre homologique. Tohoku Mathematical Journal, 9(2), 119-221.
Deligne, P. (1970). Equivariant cohomology and the equivariant de Rham cohomology. Invent. Math., 9(2), 129-144.
Quillen, D. (1973). Higher algebraic K-theory. Algebraic K-theory, I: Higher K-theories, 85-147.

Appendix

The following is a list of key terms and definitions used in this article.

Equivariant algebraic de-Rham cohomology: The cohomology of the complex of differential forms on the quotient stack $[X/G]$ .
Quotient stack: The stack $[X/G]$ is the quotient of the algebraic variety $X$ by the group $G$ .
Differential forms: The differential forms on an algebraic variety $X$ are the sections of the sheaf $\Omega^{\bullet}_X$ .
G-action: The $G$ -action on an algebraic variety $X$ is a group action of $G$ on $X$ .
Grothendieck theorem: The theorem of Grothendieck that establishes an equivalence between the algebraic de-Rham cohomology and the singular cohomology of the complex points of an algebraic variety.