The concept of separable algebra is a strengthening of the concept of semisimple algebra, and a generalization of the concept of a separable field extension.
There are a several equivalent characterizations of separable algebras. For all of these we fix a field $k$. In what follows, all $k$-algebras will be assumed associative and unital.
First, a $k$-algebra $A$ is defined to be separable if for every field extension $K$ of $k$, the algebra $A \otimes_k K$ is semisimple.
Second, a $k$-algebra $A$ is separable if and only if it is flat when considered as a right module of $A^e = A \otimes_k A^{op}$ in the obvious (but perhaps not quite standard) way.
Third, a $k$-algebra $A$ is separable if and only if it is projective when considered as a left module of $A^e$ in the usual way.
Fourth, a $k$-algebra $A$ is separable if and only if the $A^e$-module morphism
has a right inverse, that is a $A^e$-module morphism
with $m f = 1_A$.
It is easy to see that the third and fourth definitions are equivalent. We have an epimorphism of $A^e$-modules
If $f$ as above exists, this splits, so $A$ is a summand of a free $A^e$-module, namely $A^e$ itself, so $A$ is projective as an $A^e$-module. Conversely, if $A$ is projective, any epimorphism to $A$ splits.
We can also state the fourth characterization in a more grungy way in terms of the element $p = f(1)$. Namely, a $k$-algebra $A$ is separable if and only if there exists an element
such that
and
for all $a \in A$. Such an element $p$ is called a separability idempotent, since it satisfies $p^2 = p$. While grungy, finding a separability idempotent is a practical way to prove that an algebra is separable.
There is a classification theorem for separable algebras: separable algebras are the same as finite products of matrix algebras over division algebras whose centers are finite dimensional separable field extensions of the field $k$.
A perfect field is one for which every extension of is separable. Examples include fields of characteristic zero, or finite fields, or algebraically closed fields, or extensions of perfect fields. If $k$ is a perfect field, separable algebras are the same as finite products of matrix algebras over division algebras whose centers are finite-dimensional field extensions of the field $k$. In other words, if $k$ is a perfect field, there is no difference between a separable algebra over $k$ and a finite-dimensional semisimple algebra over $k$.
A result of Eilenberg and Nakayama that any separable algebra over a field $k$ can be given the structure of a symmetric Frobenius algebra. Since the underlying vector space of a Frobenius algebra is isomorphic to its dual, any Frobenius algebra is necessarily finite dimensional, and so the same is true for separable algebras. For more details, see:
A separable algebra is said to be strongly separable if there exists a separability idempotent $p$ that is symmetric, meaning
An algebra is strongly separable if and only if it can be made into a special Frobenius algebra. When this can be done, it can be done in a unique way.
There is an equivalent characterization of strongly separable algebras which makes this fact clearer. Any element $a$ of an associative unital algebra gives a left multiplication map
When $A$ is finite-dimensional, there is a bilinear pairing $g: A \times A \to k$ defined by
An algebra $A$ is strongly separable if and only if $g$ is nondegenerate, i.e., if there is an isomorphism $A \to A^*$ given by
In this case, there is just one way to make $A$ into a special Frobenius algebra, namely by defining the counit to be
Here are some examples of strongly separable algebras:
the algebra of $n \times n$ matrices with entries in the field $k$ is strongly separable if and only if $n$ is not divisible by the characteristic of $k$.
the group algebra $k[G]$ of a finite group is strongly separable if and only if the order of $G$ is not divisible by the characteristic of $k$.
For more details, see Aguiar’s book below.
More generally, if $k$ is any unital commutative ring, we can define a separable $k$-algebra to be an algebra $A$ such that $A$ is projective as a module over $A^e = A \otimes_k A^{op}$.
As in the case of algebras over a field, an algebra $A$ over a commutative ring $k$ is separable if and only if the $A^e$-module epimorphism
splits, and this in turn is equivalent to the existence of a separability idempotent.
If a separable algebra $A$ is also projective as a module over $k$, it must be finitely generated as a $k$-module. For more details see DeMeyer-Ingraham.
The ring extension $S$ over $R$ is said to be a separable extension if all short exact sequences of $S$-$S$-bimodules that are split as $S$-$R$-bimodules also split as $S$-$S$-bimodules. This is equivalent to the statement that the relative Hochschild cohomology $HH^n(S,R;M) = 0$ for all $n\gt 0$ and all coefficient bimodules $M$.
Commutative separable algebras are important in algebraic geometry. The concept of étale cover in algebraic geometry is sort of a combination of covering space and separable algebra business. Lieven Le Bruyn has written “in categorical terms, studying the monoidal cat of commutative separable $k$-algebras is the same as studying the étale site of $k$”. This stuff would be nice to make precise…
Separable algebras play a major role in the Galois theory of extensions of algebras. Every separable $k$-algebra is a filtered colimit of finite-dimensional separable $k$-algebras???
There are further generalizations, leading to separable functors…
$n$Lab: separable functor, separable field extension, separable coring
wikipedia separable algebra
Marcelo Aguiar, A note on strongly separable algebras, Boletín de la Academia Nacional de Ciencias (Córdoba, Argentina), special issue in honor of Orlando Villamayor, 65 (2000) 51-60. (web)
F. DeMeyer and E. Ingraham, Separable algebras over commutative rings, Lecture Notes in Mathematics 181, Springer, Berlin, 1971.
K. Hirata, K. Sugano, On semisimple and separable extensions of noncommutative rings, J. Math. Soc. Japan 18 (1966), 360-373.
An explicit proof of the Grothendieck Galois theory statement that the category of separable algebras over a field $K$ is anti-equivalent to the category of continuous actions on finite sets of the profinite fundamental group of $K$: