Let $k$ be the field $F\_2((X,Y))$, where $F\_2$ is the field with two elements and
$X$ and $Y$ are two indeterminates. Can we describe the Brauer group of $k$, or at least its $2$-torsion? 


(My motivation is as one could expect: I have an irreducible representation of a group of dimension 2 over the separable closure of $k$, whose trace is in k, and I am trying to determine whether or not it is realizable over $k$)



Brauer group of a field of power series in two variables.
You can get a fairly good picture of the elements of order $2$ of the Brauer
group in the following way. There is no reason to fixate on characteristic $2$
so I assume that we are dealing with $K:=\mathbb F\_p((X,Y))$ and in fact the only
reason to stick to the prime field is notational convenience as the Frobenius
map on $\mathbb F\_p$ is the identity so the relative Frobenius is equal to the
absolute one which means that I won't have to distinguish between $Z$ and
$Z^{(p)}$. Technically it may be that proper references would require $Z$ to be
of finite type over the base but everything I say will be clearly true also for
$Z=\text{Spec}\mathbb F\_p((X,Y))$. (On the other hand the only thing I will need
of $Z$ except for a smoothness/regularity assumption is that it is affine and
with trivial Picard group).


We define the sheaf (on the small étale site of $Z$) $\nu$ by the exact sequence
$$
0\rightarrow\mathcal O\_Z^\ast\xrightarrow{p}O\_Z^\ast\rightarrow\nu\rightarrow0.
$$
We have a map $\text{dlog}\colon\mathcal O\_Z^\ast\rightarrow\Omega^1\_Z$, where
$\text{dlog}(f):=df/f$ and it factors to give an injection $\nu\subseteq
\Omega^1\_Z$. More precisely, it lands in the subsheaf $Z^1$ of closed forms and we
have an exact sequence (again on the small étale site):
$$
0\rightarrow\nu\rightarrow Z^1\xrightarrow{C-\iota}\Omega^1\_Z\rightarrow0,
$$
where $C\colon Z^1/B^1\rightarrow\Omega^1\_Z$, is the Cartier isomorphism ($B^1$
being the exact $1$-forms) and $\iota\colon Z^1\subseteq \Omega^1\_Z$ is the
inclusion.


Now, the first sequence (and the fact that $\text{Pic}(Z)=0$) gives that
$H^1(Z,\nu)$ is the kernel of multiplication by $p$ on the Brauer group. The
fact that $Z$ is affine gives that $H^1(Z,Z^1)=0$ and hence the second sequence
gives that $H^1(Z,\nu)$ is the cokernel of $C-\iota\colon H^0(Z,Z^1)\rightarrow
H^0(Z,\Omega^1\_Z)$. This cokernel can be made very explicit (and to make it very
explicit we temporaritly assume $p=2$):


$H^0(Z,Z^1)$ is a module over $K$, where scalar multiplication is given by the
square map $f\cdot\omega=f^2\omega$, and has a basis given by $dX$, $dX/X$,
$d(XY)$, $dY$ and $dY/Y$. We have that $C$ is $0$ on $dX$, $d(XY)$ and $dY$ and
$C(dX/X)=dX/X$ and $C(dY/Y)=dY/Y$. Furthermore, $C$ is linear in the sense that
$C(f^2\omega)=fC(\omega)$. This implies that the relations in the cokernel are
given by $f^2dX=0$, $f^2dY=0$,
$f^2XY(dX/X+dY/Y)=0$, $(f^2-f)dX/X=0$ and $(f^2-f)dY/Y=0$ (where $dX$, $dY$,
$dX/X$, $dY/Y$, $XdY$ and $YdX$ is a $K$-basis for $H^0(Z,\Omega^1\_Z)$). This
allows for a fairly transparent normal form for elements in $H^1(Z,\nu)$.


If one wants a direct description of the central simple algebra associated to an
element $\omega\in H^0(Z,\Omega^1\_Z)$ one can apply the What Else Can It Be-principle
(a very useful though somewhat dangerous principle, in this case it is probably
OK). Recall that we have the algebra $\mathcal D$ of differential operators of order $<p$. It
is the ring generated by derivations and elements of $K$ with the relations
$DE-ED=[D,E]$, $Df-fD=D(f)$ and $D^p=D^{[p]}$, where $D^{[p]}$ is the $p$'th
power as derivation. Note that the center consists of $K^p$, the subfield of
$p$'th powers of $K$ and $K$ is in a natural fashion a $\mathcal D$-module
giving an isomorphism $\mathcal D\rightarrow\text{End}\_{K^p}(K)$ so that it is a
central simple algebra (over $K^p$ which however is isomorphic to $K$).


We can then twist this by $\omega$ by replacing the last
relation with $D^p=D^{[p]}+\omega(D)^p$. This still gives a central simple
algebra and it should (as I said according to the WECIB-principle) be the
associated element of ${}\_p\text{Br}(K)$.


There is however a different way of getting explicit representatives. For this
we realise instead ${}\_p\text{Br}(K)$ as $H^2(Z,\mu\_p)$ (now in the flat
topology). We then have the usual cup product map $H^1(Z,\mathbb Z/p)\bigotimes
H^1(Z,\mu\_p)\rightarrow H^2(Z,\mu\_p)$. We can represent elements of
$H^1(Z,\mathbb Z/p)$ by Artin-Schreier extensions $b^p-b=a$, where $a\in K$ and
elements of $H^1(Z,\mu\_p)$ by $p$'th root extensions $g^p=f$, where $f\in
K^\ast$. The central simple algebra associated to the cup product of these two
classes is the algebra generated by $K(b)$ and $g$ and relations $gbg^{-1}=b+1$
and $g^p=f$. On the other hand a straightforward computation shows that the
class of the cup product in $H^1(Z,\nu)$ is the residue of $adf/f\in
H^0(Z,\Omega^1\_Z)$. As $H^0(Z,\Omega^1\_Z)$ is generated as a group by such
elements we get a different description of the class of ${}\_p\text{Br}(K)$
associated to elements of $H^0(Z,\Omega^1\_Z)$. Note however, that they will not
be isomorphic as algebras, the algebra associated by the first procedure to
$adf/f$ has $K$-dimension $p^4$ whereas the second construction has dimension $p^2$.


