Brauer group ($\mathrm{Br}(k)$)

The Brauer group of a field $k$ consists of Morita equivalence classes of central simple $k$-algebras. Multiplication is induced by tensor product, the identity is $[k]$, and the inverse of $[A]$ is the class of the opposite algebra $A^{\mathrm{op}}$.

The Brauer class of a central simple algebra is its equivalence class under adjoining matrix-algebra factors. Two algebras have the same class exactly when their central division-algebra representatives are isomorphic. The zero class consists of split matrix algebras; the inverse of $[A]$ is the class of the opposite algebra. For a cyclic algebra $(L/K,\sigma ,a)$, the class is zero exactly when $a$ is a field norm from $L$.

For a number field $K$, localization of the Brauer group is injective. Its image is exactly the finite-support tuples of local classes whose local Brauer invariants sum to zero. The non-Archimedean summands have invariant group $\mathbb{Q}/\mathbb{Z}$, real summands have $\{0,1/2\}$, and complex summands are zero. For a cyclic character $\chi$ and parameter $a$, the invariant of the local cyclic algebra is $\chi ({\mathrm{Art}}_{K_v}(a))$. Thus the zero-sum condition on principal parameters is the additive form of Artin reciprocity.

Associate to each prime $p\equiv 3(\mathrm{mod}4)$ the class of $(-1,p{)}_{\mathbb{Q}}$. These classes have order two, since quaternion conjugation identifies the algebra with its opposite. The product rule $[(-1,a)][(-1,b)]=[(-1,ab)]$ follows by factoring their tensor product into the commuting subalgebras generated by $i_1,j_1{j}_2$ and by $i_1{i}_2,j_2$; the second is the split algebra $(1,b)$. Every nonempty finite product of distinct such primes has an odd valuation at one of them. The quaternion norm obstruction at primes congruent to three modulo four therefore makes its class nonzero, proving independence. Euclid's construction $4p_1\cdots p_m-1$ proves there are infinitely many such primes.

In characteristic $p$, the exact sequence $1\to K_s^{\times }\stackrel{p}{\to }{K}_s^{\times }\to K_s^{\times }/(K_s^{\times }{)}^p\to 1$ has a quotient killed by $p$. The characteristic p Galois dimension bound makes its second cohomology zero. Hence multiplication by $p$ on $\mathrm{Br}(K)$ is surjective. The root map is injective, and its failure of surjectivity on a separable closure is encoded by the quotient.

For a complete discretely valued field $K$ with perfect residue field $k$, there is a split exact sequence $0\to \mathrm{Br}(k)\to \mathrm{Br}(K)\stackrel{{\partial }_K}{\to }{H}^1(k,\mathbb{Q}/\mathbb{Z})\to 0$. The residue is canonical. A uniformizer chooses a splitting $\chi \mapsto (\chi ,\pi )$; this choice does not make the residue itself noncanonical.

For a local field with finite residue field, evaluate the Witt residue character on arithmetic Frobenius. This gives ${\mathrm{inv}}_K:\mathrm{Br}(K)\cong \mathbb{Q}/\mathbb{Z}$. For the unramified degree-$m$ cyclic algebra with arithmetic Frobenius generator and parameter $a$, the invariant is $v_K(a)/m$. Its reduced denominator is the index of the division representative.

For a finite Galois extension $L/K$ of non-Archimedean local fields, its relative Brauer group is the kernel of restriction $\mathrm{Br}(K)\to \mathrm{Br}(L)$ and identifies with $H^2(\mathrm{Gal}(L/K),L^{\times })$. Restriction multiplies the local Brauer invariant by $[L:K]$. This kernel is therefore cyclic of order $[L:K]$, and the indicated class is its distinguished generator. In an unramified extension with arithmetic Frobenius generator it is represented by the cyclic algebra with parameter a uniformizer.

For a finite extension of local fields $L/K$, restriction multiplies the local Brauer invariant by $[L:K]$ and corestriction preserves it. For restriction, the residue-character value multiplies by the residue degree and the parameter valuation by the ramification index. For corestriction, lift the residue character using divisibility of $\mathbb{Q}/\mathbb{Z}$ and use the norm projection formula for cyclic Brauer pairings.

The period is the order of a class in the Brauer group. It divides its index. Over a local field with finite residue field, the period and index are both the reduced denominator of its local Brauer invariant.

If $A\cong M_r(D)$ with $D$ a central division algebra, its index is $\mathrm{deg}D$. It depends only on the Brauer group class. The index divides every finite splitting-field degree and equals the degree of a maximal subfield of the division representative.

For a finite Galois extension $L/k$, Galois group $G$, and normalized two-cocycle $\varphi :G\times G\to L^{\times }$, the crossed-product algebra iswith $u_{\sigma }a=\sigma (a)u_{\sigma }$ and $u_{\sigma }{u}_{\tau }=\varphi (\sigma ,\tau )u_{\sigma \tau }$. It is a central simple $k$-algebra split by $L$.

For a cyclic extension $E/K$ of degree $m$ with generator $\sigma$ and $a\in K^{\times }$, the cyclic algebra is generated by $E$ and $u$ with $ub=\sigma (b)u$, $u^m=a$. It is a central simple algebra of degree $m$. Its Brauer class can also be written $(\chi ,a)$, where $\chi (\sigma )=1/m$.

If $a=N_{L/K}c$, let $L$ act on itself by multiplication and let the extra generator act by $c\sigma$. Its $n$th power is multiplication by $a$, and these operators give the cyclic algebra as ${\mathrm{End}}_K(L)$. Conversely, if the algebra is a matrix algebra on an $n$-dimensional vector space, its embedded degree-$n$ field $L$ makes that space one-dimensional over $L$. The extra generator is then $c\sigma$ in an $L$-basis, so its $n$th power forces $a=Nc$.

For a finite extension $L/K$, an unramified or general cyclic character $\chi$ over $K$, and $a\in L^{\times }$, transfer satisfies ${\mathrm{Cor}}_{L/K}(\mathrm{Res}\chi ,a)=(\chi ,N_{L/K}a)$. It is the cup-product projection formula combined with the field norm on multiplicative degree-zero coefficients. With the norm definition of Brauer transfer it also covers purely inseparable steps.

Cohomologous normalized two-cocycles define isomorphic crossed-product algebras: replacing $\varphi$ by the coboundary associated with a one-cochain rescales the basis elements $u_{\sigma }$. Consequently the crossed-product construction induces a map