Documentation

Mathlib.Algebra.Central.Defs

Central Algebras #

In this file we define the predicate Algebra.IsCentral K D where K is a commutative ring and D is a (not necessarily commutative) K-algebra.

Main definitions #

Implementation notes #

We require the K-center of D to be smaller than or equal to the smallest subalgebra so that when we prove something is central, we don't need to prove ⊥ ≤ center K D even though this direction is trivial.

Central Simple Algebras #

To define central simple algebras, we could do the following:

class Algebra.IsCentralSimple (K : Type u) [Field K] (D : Type v) [Ring D] [Algebra K D] where
  [is_central : IsCentral K D]
  [is_simple : IsSimpleRing D]

but an instance of [Algebra.IsCentralSimple K D] would not imply [IsSimpleRing D] because of synthesization orders (K cannot be inferred). Thus, to obtain a central simple K-algebra D, one should use Algebra.IsCentral K D and IsSimpleRing D separately.

Note that the predicate Algebra.IsCentral K D and IsSimpleRing D makes sense just for K a CommRing but it doesn't give the right definition for central simple algebra; for a commutative ring base, one should use the theory of Azumaya algebras. In fact ideals of K immediately give rise to nontrivial quotients of D so there are no central simple algebras in this case according to our definition, if K is not a field. The theory of central simple algebras really is a theory over fields.

Thus to declare a central simple algebra, one should use the following:

variable (k D : Type*) [Field k] [Ring D] [Algebra k D]
variable [Algebra.IsCentral k D] [IsSimpleRing D]
variable [FiniteDimensional k D]

where FiniteDimensional k D is almost always assumed in most references, but some results do not need this assumption.

Tags #

central algebra, center, simple ring, central simple algebra

source
class Algebra.IsCentral (K : Type u) [CommSemiring K] (D : Type v) [Semiring D] [Algebra K D] :
Prop

For a commutative ring K and a K-algebra D, we say that D is a central algebra over K if the center of D is the image of K in D.

Instances