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Central simple algebra

A central simple algebra over a field k is a finite-dimensional associative k-algebra A that is simple (i.e., has no nontrivial two-sided ideals) and has center exactly equal to k.^[1]^[2] By Wedderburn's theorem, every central simple algebra is isomorphic to a full matrix algebra M_n(D) over a central division algebra D finite-dimensional over k, where the degree of A (the square root of its dimension over k) equals n \cdot \deg(D).^[1]^[2] The dimension of a central simple algebra over k is always a perfect square.^[1] Central simple algebras are classified up to Brauer equivalence (where A \sim B if A \otimes_k M_m(k) \cong B \otimes_k M_n(k) for some m, n) by the Brauer group \Br(k), which forms an abelian torsion group under the operation [A] + [B] = [A \otimes_k B] and is often computed via Galois cohomology as

H^2(\Gal(k^s/k), k^s^\times)

, where k^s is a separable closure of k.^[1]^[2] Prominent examples include full matrix algebras M_n(k) (which are split, i.e., Brauer trivial) and quaternion algebras (a,b)_k = k \langle i,j \rangle with relations i^2 = a, j^2 = b, ji = -ij for a,b \in k^\times, such as the Hamilton quaternions over \mathbb{R}.^[1] More generally, cyclic algebras (a,\sigma)_n arise from a Galois extension with cyclic Galois group generated by \sigma and element a, providing generators for torsion in the Brauer group when k contains suitable roots of unity.^[2] Central simple algebras play a central role in noncommutative algebra, with key theorems like the Skolem-Noether theorem (asserting that k-algebra automorphisms of simple algebras are inner) and the double centralizer theorem facilitating their study.^[1] Their theory connects deeply to number theory, algebraic geometry (via Severi-Brauer varieties, which are projective varieties classifying central simple algebras of given degree), and K-theory, with applications in class field theory and the study of Galois cohomology.^[2]

Definition and Examples

Definition

An associative algebra over a field K is a vector space A over K equipped with a bilinear multiplication map \mu: A \times A \to A that is associative, meaning (\mathbf{a} \mathbf{b}) \mathbf{c} = \mathbf{a} (\mathbf{b} \mathbf{c}) for all \mathbf{a}, \mathbf{b}, \mathbf{c} \in A, and typically includes a unit element, though the presence of a unit is sometimes specified separately.^[3]^[4] A two-sided ideal in an associative algebra A is a subspace I \subseteq A such that A I \subseteq I and I A \subseteq I. An associative algebra A is called simple if its only two-sided ideals are \{0\} and A itself.^[5] The center of an associative algebra A, denoted Z(A), consists of all elements z \in A that commute with every element of A, i.e., Z(A) = \{ z \in A \mid z a = a z \ \forall a \in A \}.^[6] A central simple algebra over a field K is a finite-dimensional associative K-algebra A (i.e., \dim_K A < \infty) that is simple and central, meaning its center Z(A) is exactly K.^[7]^[8]

Basic Examples

The full matrix algebra M_n(K) over a field K, consisting of all n \times n matrices with entries in K, is a central simple algebra over K of degree n, as it is simple and has center precisely K.^[9] These algebras serve as the split examples of central simple algebras, illustrating the general structure where non-split cases resemble matrix algebras over division rings.^[10] Finite-dimensional central division algebras over a field K provide non-split examples of central simple algebras, where every nonzero element is invertible and the center is exactly K. A classical instance is the quaternion algebra \mathbb{H} over the real numbers \mathbb{R}, which has dimension 4 and basis \{1, i, j, k\} satisfying i^2 = j^2 = -1 and k = ij.^[11] Over \mathbb{R}, \mathbb{H} is the unique non-trivial central division algebra up to isomorphism.^[9] The field K itself is a trivial central simple algebra over K of degree 1, as it is simple with center K. For example, the complex numbers \mathbb{C} form a central simple algebra over \mathbb{C}, though over \mathbb{R} they constitute a division algebra that is simple but not central, since their center is \mathbb{C} rather than \mathbb{R}.^[12] Cyclic algebras offer a general construction of central simple algebras over a field K containing a cyclic extension F/K of degree n, given by A_{n,\sigma}(K, F, a) = K \langle t \rangle / (t^n = a, \, t x t^{-1} = \sigma(x) \ \forall x \in F), where \sigma generates the Galois group of F/K and a \in K^\times. These algebras have dimension n^2 over K and are central simple.^[9]

Core Properties

Structure and Isomorphism

A central simple algebra A over a field k has a canonical structure given by the Artin–Wedderburn theorem: A is isomorphic to the algebra of r \times r matrices M_r(D) over a central division algebra D over k, where r \geq 1 is an integer known as the matrix size of A. This decomposition captures the internal ring structure of A, reducing the study of such algebras to that of their underlying division components. For instance, the full matrix algebra M_n(k) over k corresponds to the case where D = k and r = n. The integers r and the isomorphism class of D are uniquely determined by A. Specifically, if A \cong M_r(D) \cong M_s(E) for central division algebras D, E over k, then r = s and D \cong E as k-algebras. This uniqueness ensures that A is structurally determined by the pair (r, [D]), where [D] denotes the Brauer class of D; thus, every central simple algebra is classified up to isomorphism by its associated Brauer class together with the matrix size r. The opposite algebra A^{\mathrm{op}}, obtained by reversing the multiplication operation so that a \cdot_{\mathrm{op}} b = ba for a, b \in A, is also a central simple algebra over k. In particular, A^{\mathrm{op}} \cong M_r(D^{\mathrm{op}}), where D^{\mathrm{op}} is the opposite of D, and the Brauer class of A^{\mathrm{op}} is the additive inverse of that of A in the Brauer group of k. Since A is a finite-dimensional simple algebra over the field k, it contains no zero divisors. If a \neq 0 and there exists b \neq 0 such that ab = 0, then the principal left ideal aA would be a proper nonzero left ideal of A, contradicting the simplicity of A.

Dimension Characteristics

Central simple algebras over a field k are finite-dimensional vector spaces over k, and their dimension \dim_k A is always a perfect square, specifically \dim_k A = n^2 for some positive integer n. This integer n, called the degree of A and denoted \deg(A), is an invariant of the algebra that remains unchanged under Brauer equivalence and base field extensions.^[2]^[13] By the Artin-Wedderburn theorem, every central simple algebra A over k is isomorphic to a matrix algebra M_r(D) over a central division algebra D with center k, where r \geq 1 is an integer and D is unique up to isomorphism. In this decomposition, the degree satisfies \deg(A) = r \cdot \deg(D), where \deg(D) = \sqrt{\dim_k D}. The index of A, denoted \ind(A), is defined as \ind(A) = \deg(D), which divides \deg(A) and measures the "division part" of the algebra's structure.^[2]^[13] Associated to A, there are canonical maps generalizing the determinant and trace from matrix algebras: the reduced norm \Nrd_A: A \to k, which is multiplicative, and the reduced trace \Trd_A: A \to k, which is additive k-linear. These are defined via the left regular representation: for a \in A, \Nrd_A(a) is the determinant of the k-linear map A \to A given by left multiplication by a, and \Trd_A(a) is the trace of that map. An element a \in A is invertible if and only if \Nrd_A(a) \neq 0. For the real quaternion algebra H = (\mathbf{i}, \mathbf{j})_{\mathbb{R}} with basis \{1, \mathbf{i}, \mathbf{j}, \mathbf{k}\} and relations \mathbf{i}^2 = -1, \mathbf{j}^2 = -1, \mathbf{k} = \mathbf{i}\mathbf{j}, the reduced norm of a general element t + x\mathbf{i} + y\mathbf{j} + z\mathbf{k} (with t,x,y,z \in \mathbb{R}) is \Nrd_H(t + x\mathbf{i} + y\mathbf{j} + z\mathbf{k}) = t^2 + x^2 + y^2 + z^2, while the reduced trace is \Trd_H(t + x\mathbf{i} + y\mathbf{j} + z\mathbf{k}) = 2t.^[2]^[14] For a field extension E/k, the Schur index m_A(E) of A relative to E is the minimal positive integer m such that A \otimes_k E admits an E-linear representation of degree m, or equivalently, such that M_m(E) is similar to a direct summand of A \otimes_k E. This index depends only on the Brauer class of A and divides \deg(A). Over a splitting field E/k (where A \otimes_k E \cong M_{\deg(A)}(E)), the Schur index simplifies to m_A(E) = 1.^[2]

Splitting and Equivalence

Splitting Fields

A splitting field for a central simple algebra A over a field k is a field extension E/k such that A \otimes_k E \cong M_n(E), where n = \deg(A) = \sqrt{\dim_k A} is the degree of A.^[15] This condition implies that A becomes isomorphic to a full matrix algebra over E, meaning it is "split" or matrix-like in this extension.^[15] The notion of splitting fields characterizes the behavior of central simple algebras under base change and plays a fundamental role in their classification.^[15] The minimal degree of a splitting field extension for A over k is given by the Schur index m_A(k), which equals the index of A and divides \deg(A).^[15] More precisely, the index of A is the greatest common divisor of the degrees of all finite separable splitting fields of A.^[15] This minimal splitting degree measures the "difficulty" of splitting A and is invariant under certain base changes.^[15] Every central simple algebra A over k admits a splitting field; in fact, it has a separable splitting field, and hence also a finite Galois splitting field.^[15] For instance, the separable closure of k splits every central simple algebra over k.^[15] This existence follows from the structure theorem for central simple algebras and properties of field extensions.^[15] Examples of splitting fields abound in classical cases. The Hamilton quaternions \mathbb{H} over \mathbb{R} have Schur index 2 and are split by any quadratic extension of \mathbb{R}, such as \mathbb{C}/\mathbb{R}, via \mathbb{H} \otimes_\mathbb{R} \mathbb{C} \cong M_2(\mathbb{C}).^[15] For cyclic algebras (L/k, \sigma, b), where L/k is a cyclic Galois extension of degree m with Galois group generated by \sigma, the defining extension L itself serves as a splitting field.^[15]

Tensor Equivalence and Classes

Central simple algebras over a field K are closed under tensor products. Specifically, if A and B are central simple K-algebras, then A \otimes_K B is also a central simple K-algebra. The dimension of A \otimes_K B over K equals the product of the dimensions of A and B over K. This closure property allows the tensor product to define a binary operation on the set of isomorphism classes of central simple algebras, facilitating the study of their classification up to equivalence.^[16] A fundamental notion in this context is Brauer equivalence, which groups central simple algebras into classes based on their splitting behavior. Two central simple K-algebras A and B are Brauer equivalent, denoted A \sim B, if A \otimes_K B^{\mathrm{op}} is isomorphic to a matrix algebra M_m(K) for some positive integer m, where B^{\mathrm{op}} denotes the opposite algebra of B (with multiplication reversed). This relation is reflexive, symmetric, and transitive, hence an equivalence relation on the isomorphism classes of central simple K-algebras. The equivalence class of A is denoted [A], and Brauer equivalent algebras share essential structural features, such as being split by the same field extensions.^[17] The equivalence relation supports a cancellation property, ensuring uniqueness in tensor products with split algebras. If C is a split central simple K-algebra (i.e., C \cong M_n(K) for some n \geq 1) and A \otimes_K C \cong B \otimes_K C, then A \cong B. This holds because tensoring with a matrix algebra over K yields A \otimes_K M_n(K) \cong M_n(A), and an isomorphism between such forms implies the underlying algebras are isomorphic, preserving the division algebra component via the Artin-Wedderburn structure theorem. This cancellation is crucial for identifying when tensor products distinguish non-equivalent classes.^[1] Brauer equivalent central simple algebras exhibit identical invariants related to their size and complexity. In particular, they have the same index, which is the degree of the unique (up to isomorphism) division algebra D such that A \cong M_r(D) for some r. The degree of such an algebra, defined as the square root of the dimension over K (so \deg(A) = \sqrt{\dim_K A}), is then r \cdot \ind(A). These invariants remain unchanged under equivalence because the tensor product with the opposite algebra yields a split form, aligning the dimensional and structural properties.

Advanced Structures

The Brauer Group

The Brauer group of a field K, denoted \mathrm{Br}(K), consists of the set of equivalence classes [A] of finite-dimensional central simple algebras over K, where equivalence is defined via similarity: two algebras A and B represent the same class if there exist integers m, n \geq 1 such that A \otimes_K M_m(K) \cong B \otimes_K M_n(K). The group operation is given by [A] + [B] = [A \otimes_K B], with the inverse [A]^{-1} = [A^\mathrm{op}], where A^\mathrm{op} is the opposite algebra; this structure forms an abelian group under the tensor product over K.^[18]^[19] The trivial element in \mathrm{Br}(K) is the class of any split algebra, such as [M_n(K)] = 0 for all n \geq 1, since matrix algebras over K are Brauer equivalent to the base field itself. This equivalence relation builds on the tensor product operation introduced earlier, classifying central simple algebras up to Morita equivalence in the category of algebras over K.^[18]^[19] There is a canonical isomorphism \mathrm{Br}(K) \cong H^2(\mathrm{Gal}(\bar{K}/K), \bar{K}^\times), where \bar{K} denotes a separable closure of K, identifying the Brauer group with the second Galois cohomology group of the absolute Galois group of K with coefficients in the multiplicative group of \bar{K}. This cohomological description arises from interpreting central simple algebras as twisted forms of matrix algebras via Galois actions on their splitting fields.^[20]^[21] For any field K, \mathrm{Br}(K) is a torsion group, meaning every element has finite order. In particular, when K is a global field, \mathrm{Br}(K) is torsion; when K is a local field, there is an explicit isomorphism \mathrm{Br}(K) \cong \mathbb{Q}/\mathbb{Z}.^[19]^[22]^[23]

Index and Period Invariants

In the Brauer group \operatorname{Br}(K) of a field K, the period of a class [A], where A is a central simple algebra over K, is defined as the minimal positive integer m such that m[A] = 0 in \operatorname{Br}(K), or equivalently, such that the tensor power A^{\otimes m} is split by K.^[20] This order reflects the torsion nature of the group, as the period is always finite.^[20] The index of [A] is the square root of the dimension of the division algebra D Brauer-equivalent to A, representing the minimal degree of a splitting field extension.^[20] A fundamental relation holds: the period divides the index, and moreover, the period and index share the same prime factors.^[20] Equality between period and index occurs in many cases, such as over local fields, where every class has period equal to its index.^[24] The period-index problem seeks explicit bounds on the index in terms of the period, addressing how the minimal splitting degree relates to the order in the Brauer group. For number fields, the Albert-Brauer-Hasse-Noether theorem implies that the index equals the period, so the index divides the period to the power 1.^[24] In broader contexts, such as fields of finite transcendence degree, the Brauer dimension measures the minimal exponent n such that the index divides the period to the power n, with known bounds like n \leq r for transcendence degree r over a perfect field in characteristic p.^[24] The n-torsion subgroup \operatorname{Br}(K) consists of classes of period dividing n. For p-adic fields \mathbb{Q}_p, the full Brauer group is isomorphic to \mathbb{Q}/\mathbb{Z}, hence torsion with all elements of finite period. The Albert-Brauer-Hasse-Noether theorem resolves a key cohomological and arithmetic aspect by stating that for a global field k, there is an exact sequence

$0 \to \operatorname{Br}(k) \to \bigoplus_{v \in \Omega} \operatorname{Br}(k_v) \xrightarrow{\sum \operatorname{inv}_v} \mathbb{Q}/\mathbb{Z} \to 0,

where \Omega is the set of places of k, k_v are the completions, and \operatorname{inv}_v are the local invariant maps; thus, every global class is a sum of local classes, bridging local and global invariants and addressing gaps in the Hasse principle for central simple algebras.^[25]

References

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A central simple algebra (CSA) over a field k is a finite dimensional k-algebra with center k and no non-trivial two-sided ideals. Some examples: • Any division ...
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