Generic matrix ring

In algebra, a generic matrix ring is a sort of a universal matrix ring.

Definition

We denote by a generic matrix ring of size n with variables . It is characterized by the universal property: given a commutative ring R and n-by-n matrices over R, any mapping extends to the ring homomorphism (called evaluation) .

Explicitly, given a field k, it is the subalgebra of the matrix ring generated by n-by-n matrices , where are matrix entries and commute by definition. For example, if m=1 then is a polynomial ring in one variable.

For example, a central polynomial is an element of the ring that will map to a central element under an evaluation. (In fact, it is in the invariant ring since it is central and invariant.[1])

By definition, is a quotient of the free ring with by the ideal consisting of all p that vanish identically on all n-by-n matrices over k.

Geometric perspective

The universal property means that any ring homomorphism from to a matrix ring factors through . This has a following geometric meaning. In algebraic geometry, the polynomial ring is the coordinate ring of the affine space , and to give a point of is to give a ring homomorphism (evaluation) (either by the Hilbert nullstellensatz or by the scheme theory). The free ring plays the role of the coordinate ring of the affine space in the noncommutative algebraic geometry (i.e., we don't demand free variables to commute) and thus a generic matrix ring of size n is the coordinate ring of a noncommutative affine variety whose points are the Spec's of matrix rings of size n (see below for a more concrete discussion.)

The maximal spectrum of a generic matrix ring

For simplicity, assume k is algebraically closed. Let A be an algebra over k and let denote the set of all maximal ideals in A such that . If A is commutative, then is the maximal spectrum of A and is empty for any .

References

  1. Artin 1999, Proposition V.15.2.
  • Artin, Michael (1999). "Noncommutative Rings" (PDF).
  • Cohn, Paul M. (2003). Further algebra and applications (Revised ed. of Algebra, 2nd ed.). London: Springer-Verlag. ISBN 1-85233-667-6. Zbl 1006.00001.
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