Executive Summary
Semiprimitivity asks for a faithful semisimple module. Tightening semisimple to simple gives left primitivity, and the resulting class of rings is the one the Density Theorem describes: every left primitive ring is a dense ring of linear transformations on a vector space over a division ring.
The corresponding notion for ideals is defined by quotienting, and identifies it concretely: the left primitive ideals of are exactly the annihilators of simple left -modules. Since is the intersection of those annihilators, follows at once — the radical is the intersection of the left primitive ideals, and equally of the right primitive ideals.
Overview
The class of left primitive rings was isolated by Jacobson in the 1940s as the correct generalisation of simple artinian. Wedderburn–Artin describes a simple left artinian ring as ; the Density Theorem describes a left primitive ring as a dense subring of for a right vector space over a division ring . When is finite the two statements coincide, so primitivity is a chain-condition-free extension of the classical theory.
Primitive rings are not rare. If is any nonzero ring and any simple left -module, then is left primitive. So every ring produces primitive quotients, one for each simple module, and recovers information about itself by mapping into their product.
The one structural surprise is that the notion has a genuine handedness. Semiprimitivity does not, because is side-neutral; primitivity does, and no amount of cleverness removes it. Bergman's construction produces a ring primitive on one side only, and further examples were later given by Jategaonkar.
Learning Objectives
- State and and check the two definitions are consistent for the zero ideal.
- Prove : left primitive ideals are precisely annihilators of simple left modules.
- Derive from and and state its right-handed twin.
- Explain why is left primitive for every simple .
- Verify that is left primitive for any nonzero right -vector space .
- Say precisely what Bergman's example shows and what it does not.
Definitions
A ring is left primitive if there exists a simple left -module with . Right primitive is defined with right modules. A ring that is both is called primitive; where the sides are known to agree — for instance under — the qualifier is often dropped.
A two-sided ideal is left primitive if the quotient ring is left primitive. In particular is a left primitive ring exactly when is a left primitive ideal of , and a left primitive ideal is always proper, since the zero ring has no simple modules.
- The two-sided ideal ; the kernel of .
- The endomorphism ring of as a left -module. By Schur's Lemma it is a division ring when is simple.
- All -linear endomorphisms of a right vector space over a division ring , acting on the left of .
- Primitive quotient
- A ring of the form with a simple left -module; always left primitive.
- Subdirect product
- A subring of a product that surjects on each coordinate. is a subdirect product of its primitive quotients precisely when it is semiprimitive.
Core Concepts
Passing between and
The entire content of is a bookkeeping identity about modules over a quotient. If , then is a module over by the obvious action, and its submodules over and over coincide; so simplicity is preserved in both directions. What changes is the annihilator, which shrinks by exactly :
Faithfulness over the quotient means the annihilator over was exactly .
So " has a faithful simple module" and " is the annihilator of a simple -module" are two readings of the same sentence.
Why the notion is handed
Left primitivity is a statement about the lattice of left ideals: is left primitive iff some maximal left ideal contains no nonzero two-sided ideal, since then is faithful. This reformulation is often the practical test.
Nothing in that criterion is symmetric: the maximal left ideals of and of are unrelated lattices. Bergman's example shows the asymmetry is real, not merely unproven. Contrast this with , which admits a description — for all — mentioning only units, and is therefore forced to be symmetric.
Primitive rings are everywhere
Let be a division ring, a nonzero right -vector space, and acting on the left. Then is simple, because a nonzero vector can be carried to any other by a -linear map, and it is faithful, because a linear map killing all of is zero. Hence is left primitive. If then is simple artinian; if is infinite, is a left primitive ring that is neither simple, nor commutative, nor left artinian.
Key Results
Let be a ring with identity and a two-sided ideal. Then is a left primitive ideal if and only if for some simple left -module .
**()** Suppose with a simple left -module. Since , the action of factors through , making a left -module with the same submodule lattice; so is simple over . Its annihilator there is , so is a faithful simple -module and is left primitive.
**()** Suppose is left primitive and let be a faithful simple left -module. Inflate along the surjection . The -submodules of are exactly its -submodules, so is simple. An element kills precisely when its class kills , that is precisely when by faithfulness; hence .
For any ring with identity, is the intersection of all left primitive ideals of , and also the intersection of all right primitive ideals. If both families are nonempty.
By , over all simple left -modules . By the ideals occurring in that intersection are precisely the left primitive ideals, so the two intersections have the same index set and agree. The right-handed statement follows by applying the same argument in , together with the left-right symmetry of established in –.
The two families of ideals may differ; their intersections cannot.
If and is any simple left -module, then is a left primitive ring. Consequently is semiprimitive if and only if embeds as a subdirect product of left primitive rings, namely its primitive quotients.
The first claim is the () half of . For the second, the natural map has kernel and is surjective in each coordinate; it is injective exactly when .
Let be a division ring and a right -vector space, . Then is a faithful simple left -module, so is left primitive. If then . If is infinite then is left primitive but not simple: the endomorphisms of finite rank form a proper nonzero two-sided ideal.
Left primitivity does not imply right primitivity. G. Bergman constructed a ring that is primitive on one side but not the other; A. V. Jategaonkar produced further families later. This is in sharp contrast to semiprimitivity, and it is why the qualifier left is carried throughout §11.
Proof Techniques and Method
How these proofs work, and which move to reuse.
Inflate and deflate
A module killed by is the same object over and over ; only the annihilator changes, and it changes by exactly . Every statement of the form "property of " translates into "property of an -module".
Largest ideal inside a left ideal
For a maximal left ideal , is the largest two-sided ideal contained in . Primitivity is the assertion that this largest ideal is zero for some .
Same intersection, different families
Two different families of ideals can have equal intersections. is exactly that phenomenon, and it is what lets a symmetric invariant be built from asymmetric pieces.
Move 2 is the version to remember when computing. To show a concrete ring is left primitive, produce a maximal left ideal and show no nonzero two-sided ideal fits inside it. That is exactly the pattern used for skew polynomial rings on the Skew Polynomial Rings as Primitive Rings page, where the ideals were first classified in so that this check became routine.
Worked Example
Primitive ideals of a commutative ring
Take . The simple modules are and , so the left primitive ideals of are exactly the maximal ideals . Their intersection is , confirming . The zero ideal is not primitive, so itself is not a primitive ring — as requires, since is commutative but not a field.
The general commutative picture: is a primitive ideal of a commutative ring iff is a field, i.e. iff is maximal. So for commutative rings, primitive ideal and maximal ideal are the same notion, and degenerates to the classical description of the Jacobson radical.
A noncommutative primitive quotient
Let be the upper triangular rational matrices, with simple modules as on the Semiprimitive Rings page. Then
and both quotients are isomorphic to , a field, hence primitive rings. So has exactly two primitive ideals; their intersection is the square-zero ideal of strictly upper triangular matrices, which is . Again checks out, and here the intersection is nonzero, so is not semiprimitive.
An infinite-dimensional primitive ring
Let be a countably infinite-dimensional right vector space over a field , . Fix a basis The module is simple: given and , extend to a basis and define a linear map sending . It is faithful since only the zero map kills every vector. Hence is left primitive.
is not simple: the finite-rank endomorphisms form a two-sided ideal with . And is not left artinian: the left ideals form a strictly descending chain. So is a left primitive ring outside the reach of Wedderburn–Artin, and it is the model case for the Density Theorem.
Frameworks and Models
Primitive rings fall into recognisable families, and knowing which family you are in determines which theorem applies.
- Left primitive rings — faithful simple left module ; is a division ring
- finite
- — simple left artinian
- recovers Wedderburn–Artin
- infinite, nonzero socle
- contains a minimal left ideal
- and its dense subrings containing finite-rank maps
- faithful simple module is unique up to isomorphism
- infinite, zero socle
- no minimal left ideals
- simple domains such as
- possibly many non-isomorphic faithful simple modules
- finite
Comparison and Classification
| Question | Ring version | Ideal version |
|---|---|---|
| Definition | has a faithful simple left module | has a faithful simple left module |
| Concrete form | some maximal left ideal contains no nonzero ideal | , simple |
| Commutative case | is a field | is a maximal ideal |
| Relation to | primitive semiprimitive | |
| Side symmetry | fails — Bergman 1965 | left and right families differ, intersections agree |
| Left primitive | Right primitive | Simple | Semiprimitive | |
|---|---|---|---|---|
| yes | yes | yes | yes | |
| , infinite | yes | yes | no | yes |
| Weyl algebra , | yes | yes | yes | yes |
| no | no | no | yes | |
| no | no | no | no | |
| Bergman's example | yes | no | no | yes |
Test cases
Relationship Map
The correspondence is surjective onto left primitive ideals but not injective: non-isomorphic simple modules can share an annihilator. The fibres of the map are studied on the Socle of a Primitive Ring page, where shows the fibre over is a single isomorphism class as soon as has a minimal left ideal.
The containment *left primitive prime* is applied to ; the containment *maximal left primitive* holds because a simple ring is primitive. Both inclusions are strict in general: is a prime but not primitive ideal of , and is a primitive but not maximal ideal of for infinite-dimensional .
Applications and Industry Use
Applications here means where this structure is used — inside mathematics and in the engineering and computing disciplines that consume it.
The primitive spectrum
For enveloping algebras of Lie algebras, the set of primitive ideals with its Jacobson topology is the basic classification invariant; Dixmier's and Duflo's work describes it for solvable and semisimple Lie algebras.
Stratification of quantum groups
Primitive ideals of quantised coordinate rings are computed by the Goodearl–Letzter stratification, transporting the ideal-theoretic questions of this page to the quantum setting.
Primitive ideal space
For a C*-algebra the primitive ideal space, kernels of irreducible representations, is the noncommutative substitute for a topological space, and is exactly read in the analytic category.
Deciding primitivity
For a finite-dimensional algebra over a field, primitivity is decidable: compute the radical, split the semisimple quotient, and test whether one simple factor accounts for the whole algebra. Beyond finite dimension, no general algorithm exists.
Honestly stated: primitive ideals are the noncommutative replacement for points of a spectrum. Wherever a commutative theory uses or the maximal spectrum, the noncommutative theory uses the primitive spectrum, and is the definition that makes that substitution work.
Standards and Notation
Standards here covers notation, symbol and markup standards, and reference implementations, rather than material or design codes.
Failure Modes and Common Mistakes
- Do not confuse with a maximal left ideal. The annihilator is two-sided and is the largest ideal inside ; it is generally much smaller than .
- Do not assume a left primitive ring has a unique faithful simple module. Uniqueness requires a minimal left ideal ; without it there can be infinitely many, as shows.
- Do not read as saying the left and right primitive ideals coincide. Only their intersections do.
- Do not expect left primitive to pass to subrings or to matrix rings for free — matrix rings do inherit primitivity, but the argument goes through Morita theory, not through the definition.
Historical Notes and Lessons Learned
- 1927Artin's chain conditionsWedderburn's structure theory for finite-dimensional algebras is extended to rings with the descending chain condition, but the arguments still lean on nilpotence and finiteness.
- 1945Jacobson's radical and primitivityJacobson defines the radical for arbitrary rings via annihilators of simple modules and isolates the primitive rings as those acting faithfully on a simple module.
- 1945–47The Density TheoremJacobson and Chevalley prove that a ring acting faithfully and irreducibly acts densely; the structure theorem for primitive rings follows.
- 1964Bergman's asymmetric ringG. Bergman constructs a ring that is primitive on one side and not on the other, settling that the notion is genuinely one-sided. Lam dates the construction to 1965.
- 1970sPrimitive spectraDixmier and others study the primitive ideals of enveloping algebras; the primitive spectrum becomes a standard classification tool, later carried into quantum algebra.
The lesson is the same as for the radical: define your invariant by how the ring acts on modules, not by what its elements look like. Jacobson's definition needs no chain condition, and the price — losing left-right symmetry — turned out to be one worth paying.
Quick Reference
| Goal | Use | Hypothesis needed |
|---|---|---|
| Show an ideal is primitive | , exhibit a simple module it annihilates | none |
| Show a ring is primitive | , exhibit a faithful simple module | none |
| Compute | none | |
| Force both sides to agree | has a minimal left ideal | |
| Get a matrix ring | left primitive and left artinian |
Frequently Asked Questions
Why introduce primitive ideals at all, when primitive rings would seem enough?
Because most rings are not primitive, but every ring has primitive ideals, and they carry the information. The primitive ideals of play the role that maximal ideals play in commutative algebra: their intersection is the radical, and the family of quotients is a supply of well-understood rings into which maps.
What exactly does Bergman's example show?
That there exists a ring with a faithful simple left module but no faithful simple right module. It does not show that the primitive ideals on the two sides are unrelated in intersection — guarantees both intersections equal . It also does not contradict , since a ring with a minimal left ideal cannot behave this way; Bergman's ring necessarily has zero socle.
Is a primitive ideal prime?
Yes. By a left primitive ring is prime, and applying that to shows every left primitive ideal is prime. The converse fails: is prime in but not primitive, because is a commutative non-field.
How do I actually test a concrete ring for left primitivity?
Find a maximal left ideal and show that the largest two-sided ideal inside , namely , is zero. In practice one first classifies the two-sided ideals of and then checks none of the nonzero ones fits inside . That is precisely the strategy of and for skew polynomial rings.
Do simple modules with the same annihilator have to be isomorphic?
No. A left primitive ring with zero socle can have many non-isomorphic faithful simple modules, all with annihilator ; with and has infinitely many. Uniqueness holds when has a minimal left ideal, by .
Is left primitive when is?
Yes. Primitivity is a Morita invariant: if is a faithful simple left -module then the column space is a faithful simple left -module. The argument is representation-theoretic rather than a direct manipulation of the definition.
References
- T. Y. Lam, A First Course in Noncommutative Rings, Graduate Texts in Mathematics 131, Springer-Verlag, 1991, §11, (11.2)–(11.5), pp. 183–185.
- N. Jacobson, Structure of Rings, American Mathematical Society Colloquium Publications 37, revised edition, 1964, Chapters I–II.
- G. M. Bergman, “A ring primitive on the right but not on the left”, Proceedings of the American Mathematical Society 15 (1964), 473–475.
- I. N. Herstein, Noncommutative Rings, Carus Mathematical Monographs 15, Mathematical Association of America, 1968, Chapter 2.
- L. H. Rowen, Ring Theory, Volume I, Academic Press, 1988, Chapter 2.
AI Suggested Questions
- Sketch Bergman's construction of a ring that is primitive on one side but not the other.
- Describe the Jacobson topology on the primitive spectrum and compute it for the first Weyl algebra.
- Show that primitivity is a Morita invariant and identify exactly which step needs the equivalence.
- For which group algebras of infinite groups is left primitive?
- Compare primitive ideals with maximal ideals in a noetherian ring satisfying a polynomial identity.
- Give a ring with exactly two primitive ideals whose intersection is nonzero, and identify its radical.
- How do the primitive ideals of a quantised coordinate ring stratify under the Goodearl–Letzter theory?
