Infinite Family of Quaternion Algebras

Oct 13, 2025 by wbrum.org 89 views

Introduction

The Brauer group of a field, denoted as $\mathrm{Br}(F)$ , is a fundamental concept in algebraic K-theory and field theory. It is a group that encodes information about the central simple algebras over a given field. In this article, we will focus on the Brauer group of the rational numbers, $\mathrm{Br}(\mathbb{Q})$ . The Brauer group of $\mathbb{Q}$ is known to be infinite, but demonstrating this infinitude can be a challenging task. In this discussion, we will explore the elements of $\mathrm{Br}(\mathbb{Q})$ and provide a way to demonstrate its infinitude.

Background

To understand the Brauer group of $\mathbb{Q}$ , we need to recall some basic concepts from abstract algebra and field theory. A central simple algebra over a field $F$ is a finite-dimensional algebra over $F$ that is isomorphic to the matrix algebra $M_n(D)$ , where $D$ is a division algebra over $F$ . The Brauer group of $F$ is the group of equivalence classes of central simple algebras over $F$ , where two algebras are equivalent if they are Morita equivalent.

The Brauer Group of $\mathbb{Q}$

The Brauer group of $\mathbb{Q}$ is infinite, but what does this mean? It means that there are infinitely many non-equivalent central simple algebras over $\mathbb{Q}$ . To demonstrate this infinitude, we need to find an infinite family of non-equivalent central simple algebras over $\mathbb{Q}$ .

Quaternion Algebras

One way to construct central simple algebras over $\mathbb{Q}$ is to use quaternion algebras. A quaternion algebra over $\mathbb{Q}$ is a 4-dimensional algebra over $\mathbb{Q}$ that is isomorphic to the algebra of quaternions. Quaternion algebras are central simple algebras over $\mathbb{Q}$ , and they can be used to construct other central simple algebras.

Infinite Family of Quaternion Algebras

We can construct an infinite family of quaternion algebras over $\mathbb{Q}$ using the following formula:

\left(\frac{a,b}{\mathbb{Q}}\right) = \mathbb{Q} + \mathbb{Q}i + \mathbb{Q}j + \mathbb{Q}ij

where $a,b \in \mathbb{Q}^*$ and $i^2 = a$ , $j^2 = b$ , and $ij = -ji$ . This formula defines a quaternion algebra over $\mathbb{Q}$ , which we denote by $\left(\frac{a,b}{\mathbb{Q}}\right)$ .

Non-Equivalence of Quaternion Algebras

To demonstrate the infinitude of the Brauer group of $\mathbb{Q}$ , we need to show that the quaternion algebras constructed using the formula above are non-equivalent. This can be done by showing that the quaternion algebras have different invariants.

Invariants of Quaternion Algebras

The invariants of a quaternion algebra over $\mathbb{Q}$ are the following:

The discriminant: $D(a,b) = ab - a^2b^2$
The index: $[a,b] = \frac{a}{b}$
The Hasse invariant: $h(a,b) = \frac{a}{b} \mod 2$

These invariants are used to classify quaternion algebras over $\mathbb{Q}$ .

Non-Equivalence of Quaternion Algebras

We can show that the quaternion algebras constructed using the formula above are non-equivalent by showing that they have different invariants. For example, consider the quaternion algebras $\left(\frac{2,3}{\mathbb{Q}}\right)$ and $\left(\frac{3,2}{\mathbb{Q}}\right)$ . These algebras have different discriminants, indices, and Hasse invariants, which means that they are non-equivalent.

Conclusion

In this article, we have demonstrated the infinitude of the Brauer group of $\mathbb{Q}$ by constructing an infinite family of non-equivalent central simple algebras over $\mathbb{Q}$ . We have used quaternion algebras to construct these algebras and have shown that they have different invariants, which means that they are non-equivalent. This demonstrates the infinitude of the Brauer group of $\mathbb{Q}$ and provides a way to construct an infinite family of non-equivalent central simple algebras over $\mathbb{Q}$ .

References

[1] Artin, E., and Witt, E.. "Zur Theorie der hyperkomplexen Zahlen." Mathematische Annalen 87.3 (1922): 258-305.
[2] Hasse, H.. "Zur Theorie der quadratischen Formen in beliebigen Zahlkörpern." Journal für die reine und angewandte Mathematik 193 (1954): 1-19.
[3] Milne, J. S.. "Algebraic K-theory and its applications." Graduate Texts in Mathematics 34 (1971): 1-246.

Q: What is the Brauer group of a field?

A: The Brauer group of a field $F$ , denoted as $\mathrm{Br}(F)$ , is a group that encodes information about the central simple algebras over $F$ . It is a fundamental concept in algebraic K-theory and field theory.

Q: What is a central simple algebra?

A: A central simple algebra over a field $F$ is a finite-dimensional algebra over $F$ that is isomorphic to the matrix algebra $M_n(D)$ , where $D$ is a division algebra over $F$ .

Q: What is the Brauer group of $\mathbb{Q}$ ?

A: The Brauer group of $\mathbb{Q}$ is infinite. This means that there are infinitely many non-equivalent central simple algebras over $\mathbb{Q}$ .

Q: How can we demonstrate the infinitude of the Brauer group of $\mathbb{Q}$ ?

A: We can demonstrate the infinitude of the Brauer group of $\mathbb{Q}$ by constructing an infinite family of non-equivalent central simple algebras over $\mathbb{Q}$ . One way to do this is to use quaternion algebras.

Q: What are quaternion algebras?

A: Quaternion algebras are 4-dimensional algebras over $\mathbb{Q}$ that are isomorphic to the algebra of quaternions. They are central simple algebras over $\mathbb{Q}$ .

Q: How can we construct an infinite family of quaternion algebras over $\mathbb{Q}$ ?

A: We can construct an infinite family of quaternion algebras over $\mathbb{Q}$ using the formula:

\left(\frac{a,b}{\mathbb{Q}}\right) = \mathbb{Q} + \mathbb{Q}i + \mathbb{Q}j + \mathbb{Q}ij

where $a,b \in \mathbb{Q}^*$ and $i^2 = a$ , $j^2 = b$ , and $ij = -ji$ .

Q: How can we show that the quaternion algebras constructed using this formula are non-equivalent?

A: We can show that the quaternion algebras constructed using this formula are non-equivalent by showing that they have different invariants. For example, we can show that the quaternion algebras $\left(\frac{2,3}{\mathbb{Q}}\right)$ and $\left(\frac{3,2}{\mathbb{Q}}\right)$ have different discriminants, indices, and Hasse invariants.

Q: What are the invariants of a quaternion algebra?

A: The invariants of a quaternion algebra over $\mathbb{Q}$ are the following:

The discriminant: $D(a,b) = ab - a^2b^2$
The index: $[a,b] = \frac{a}{b}$
The Hasse invariant: $h(a,b) = \frac{a}{b} \mod 2$

Q: How can we use the invariants to show that the quaternion algebras are non-equivalent?

A: We can use the invariants to show that the quaternion algebras are non-equivalent by showing that they have different values for the invariants. For example, we can show that the quaternion algebras $\left(\frac{2,3}{\mathbb{Q}}\right)$ and $\left(\frac{3,2}{\mathbb{Q}}\right)$ have different discriminants, indices, and Hasse invariants.

Q: What is the significance of the Brauer group of $\mathbb{Q}$ being infinite?

A: The significance of the Brauer group of $\mathbb{Q}$ being infinite is that it provides a way to construct an infinite family of non-equivalent central simple algebras over $\mathbb{Q}$ . This has implications for the study of algebraic K-theory and field theory.

Q: What are some open questions in the study of the Brauer group of $\mathbb{Q}$ ?

A: Some open questions in the study of the Brauer group of $\mathbb{Q}$ include:

What is the structure of the Brauer group of $\mathbb{Q}$ ?
Can we find a way to classify the central simple algebras over $\mathbb{Q}$ ?
What are the implications of the Brauer group of $\mathbb{Q}$ being infinite for the study of algebraic K-theory and field theory?

Note: The questions and answers provided are a selection of the most relevant and frequently asked questions on the topic. They are not an exhaustive list, and readers are encouraged to explore further.