Let $K$ be a cyclotomic field of degree $n$ and $A$ a central simple algebra over $\mathbb{Q}$ of dimension $n^2$. How can one determine whether there is a $\mathbb{Q}$-algebra embedding $K \hookrightarrow A$?


This is equivalent to asking whether $A \otimes\_{\mathbb{Q}} K \simeq M\_n(K)$. Is there a local-to-global principle that can be used?


If, for example, $A \simeq M\_n(D)$ where $D$ is a quaternion algebra over $\mathbb{Q}$, is there a simple criterion relating subfields of $K$ and splitting fields of $D$?


Any reference to a similar example where this is worked out would be greatly appreciated!



Cyclotomic fields and splitting of central simple algebras
I believe that the characterization you are after is a more or less straightforward consequence of the Albert-Brauer-Hasse-Noether theorem and recommend that you take a look at Chapter 32 of Reiner's book *Maximal Orders*. The chapter is entitled "Splitting of Simple Algebras" and covers the subject in great detail.


Let $F$ be a number field and $A$ be a finite dimensional central simple algebra over $F$. Given a prime $\mathfrak p$ of $F$ (possibly infinite) denote by $m\_\frak{p}$ the local index of $A$ at $\mathfrak{p}$ (i.e., the degree of the division algebra part of $A\otimes\_F F\_\mathfrak{p}$).


The following is Theorem 32.15 of Reiner.


**Theorem**. *Let $L$ be a finite extension of $F$. Then $L$ is a splitting field for $A$ if and only if for every prime $\mathfrak p$ of $F$ and prime $\mathfrak P$ of $L$ lying above $\mathfrak p$, we have:*
\begin{equation} 
m\_{\mathfrak p}\mid [L\_\mathfrak P:F\_\mathfrak p].
\end{equation}


