Executive Summary
Six classes of ring — simple, left primitive, prime on one row, semisimple, semiprimitive, semiprime on the other — sit in a rigid pattern of implications. Lam's supplies the two arrows that were still missing: a simple ring is left and right primitive, and a left primitive ring is both semiprimitive and prime.
The pattern is worth memorising because of what happens under hypotheses. Under left artinian, every horizontal implication reverses: prime becomes simple, semiprime becomes semisimple . Under commutative, primitivity collapses all the way to field , so the notion carries no new information there. Everything interesting about primitive rings lives in the noncommutative, non-artinian corner.
Overview
Each of the six classes is defined by killing something: ideals (simple), products of ideals (prime), nilpotent ideals (semiprime), the radical (semiprimitive), or by exhibiting a module (primitive, semisimple). The chart below is the summary of how the conditions compare.
The vertical arrows point from the bottom row to the top row; the leftmost one needs the descending chain condition.
The vertical arrow on the left is the only one requiring a hypothesis: a simple ring need not be semisimple — the Weyl algebra is simple and not artinian — but a simple left artinian ring is semisimple. The other two vertical arrows are unconditional.
The companion page Ring Class Hierarchy: Semisimple to Semilocal and Everything Between places this chart in the wider landscape, and Prime and Semiprime Rings develops the right-hand column in detail.
Learning Objectives
- Prove each of the three implications in from the definitions.
- Reconstruct the two-row chart, including which vertical arrow requires a chain condition.
- State correctly, with left artinian as the hypothesis rather than artinian.
- Prove : a commutative primitive ring is a field.
- Produce a separating example for each non-implication: prime not primitive, primitive not simple, semiprimitive not semisimple.
- Explain why makes left and right primitivity agree under a chain condition.
Definitions
- Simple
- and the only two-sided ideals are and . No chain condition is implied.
- Prime
- and for all nonzero ideals . Equivalently or .
- Semiprime
- for ideals ; equivalently .
- Left primitive
- Some simple left -module is faithful.
- Semiprimitive
- ; equivalently some semisimple left module is faithful .
- Semisimple
- is a semisimple module; equivalently and is left artinian.
All rings have an identity and are nonzero where the definitions require it. Simple rings are not assumed artinian; that assumption is what distinguishes §3 from §11.
Core Concepts
Why a simple ring is primitive
The annihilator of a module is a two-sided ideal. In a simple ring the only candidates are and , and means for a unital module. So every nonzero module over a simple ring is faithful; in particular every simple module is, and simple modules exist because maximal left ideals do.
That argument mentions no side, which is why simple rings are left and right primitive. It is also why the primitivity of a simple ring gives no structural information on its own — the content arrives only through the Density Theorem, which converts faithful simple action into a concrete matrix-like description.
Why a left primitive ring is prime
Let be a faithful simple left -module and an ideal. Then is a submodule of , and it is nonzero: otherwise . By simplicity . So every nonzero ideal acts surjectively on , and composing two surjections is a surjection — that is the entire proof.
Where the chain condition bites
For a left artinian ring, is nilpotent and is semisimple . Nilpotence of the radical is exactly what makes semiprime and semiprimitive agree: a semiprime ring has no nonzero nilpotent ideal, so a nilpotent radical must vanish. Once and is left artinian, is semisimple, hence a finite product of simple artinian rings — and a product with more than one factor is never prime.
The commutative collapse
In a commutative ring a simple module is with maximal, and its annihilator is itself, because for means . Faithfulness therefore forces , so is a field. Nothing survives: primitivity is not a useful notion for commutative rings, and the theory only becomes substantial once left ideals and two-sided ideals can differ.
Key Results
Let be a ring with identity. (a) If is simple then is both left and right primitive. (b) If is left primitive then is semiprimitive and prime.
(a) Let be simple, so and its only ideals are and . Choose a maximal left ideal , which exists by Zorn's Lemma, and set , a simple left module. Its annihilator is a two-sided ideal, and it is proper because ; hence and is faithful. So is left primitive, and the same argument with maximal right ideals gives right primitivity.
(b) Let be a faithful simple left -module. A simple module is semisimple, so gives . For primeness, let and be nonzero ideals. Since and , we have ; as is a submodule of the simple module , we get . The same for . Hence
so . As were arbitrary nonzero ideals, is prime.
Let be a left artinian ring. Then:
- is semisimple is semiprimitive is semiprime;
- is simple is left primitive is right primitive is prime.
(1) Semisimple semiprimitive for left artinian rings is . Semiprimitive semiprime is : one direction holds for every ring since a nilpotent ideal lies in ; conversely, for left artinian is nilpotent by , so semiprimeness forces .
(2) By we already have simple left (and right) primitive prime, so it suffices to show that a prime left artinian ring is simple. A prime ring is semiprime, so by part (1) is semisimple; write as a finite product of simple artinian rings. If , the ideals and are nonzero with , contradicting primeness. Hence and is simple.
A commutative ring is primitive if and only if is a field.
If is a field then itself is a faithful simple module. Conversely, let be commutative and primitive, with a faithful simple -module. Then for a maximal ideal , and because is commutative and annihilates the coset of . Faithfulness gives , so is maximal and is a field.
Combining with Wedderburn–Artin: a left artinian ring is left primitive if and only if it is isomorphic to for some and some division ring . This is the finite-dimensional shadow of the Structure Theorem .
- Semiprimitive but not semisimple: , since but is not left artinian.
- Prime but not left primitive: again — it is a domain, hence prime, but not a field, so rules out primitivity.
- Left primitive but not simple: with infinite; the finite-rank endomorphisms form a proper nonzero ideal.
- Simple but not semisimple: the Weyl algebra , simple by but not left artinian.
- Semiprime but not semiprimitive: is a domain, hence semiprime, yet .
- Left primitive but not right primitive: Bergman's example; this is the one asymmetry in the chart.
Proof Techniques and Method
How these proofs work, and which move to reuse.
Annihilators are two-sided
Any statement of the form "this module is faithful" is a statement about a two-sided ideal, so it interacts directly with simplicity, primeness and the radical. This is why module conditions produce ideal-theoretic conclusions at all.
Nonzero ideals act surjectively
On a faithful simple module, for every nonzero ideal . Chain two of these to get and primeness follows without touching elements.
Use DCC to nilpotence to zero
Left artinian gives a nilpotent radical; semiprime kills nilpotent ideals; together they force . This three-step chain is the engine of .
Move 3 explains why the reversals in are genuinely about the descending chain condition rather than about artinian-ness as a convenience. Drop the chain condition and need not be nil, so can be semiprime without being semiprimitive.
Worked Example
Working the chart on four concrete rings
**.** Simple, since the ideals of are for ideals of and is a field. Left artinian, being -dimensional over . So all six conditions hold, and the unique simple module is the column space with .
**.** Prime and semiprime (a domain), semiprimitive since , but neither simple, nor primitive , nor semisimple. It occupies the weakest cell of the chart that is not degenerate.
**, the first Weyl algebra.** Simple in characteristic , hence left and right primitive and prime and semiprimitive. It is a noetherian domain of Gelfand–Kirillov dimension and is not left artinian, so does not apply and is not semisimple. This is the standard example showing the left vertical arrow of the chart really needs DCC.
**, upper triangular matrices.** The strictly upper triangular matrices form a nonzero ideal with , so is not semiprime, hence not prime, not semiprimitive, not primitive, not simple, not semisimple. Every condition on the chart fails, and the failure is traced to a single square-zero ideal.
One square-zero ideal defeats every condition in the chart simultaneously.
Frameworks and Models
A practical way to place an unfamiliar ring on the chart is to ask three questions in order.
Does satisfy the descending chain condition on left ideals?
Comparison and Classification
| Simple | Left primitive | Prime | Semisimple | Semiprimitive | Semiprime | |
|---|---|---|---|---|---|---|
| , a division ring | yes | yes | yes | yes | yes | yes |
| , first Weyl algebra | yes | yes | yes | no | yes | yes |
| , infinite | no | yes | yes | no | yes | yes |
| no | no | yes | no | yes | yes | |
| , a field | no | no | yes | no | yes | yes |
| no | no | yes | no | no | yes | |
| no | no | no | yes | yes | yes | |
| upper triangular | no | no | no | no | no | no |
The six conditions on standard rings
| Hypothesis | Effect on the chart | Reference |
|---|---|---|
| None | Six distinct classes; only the stated arrows hold | |
| Left artinian | Both rows collapse: prime primitive simple; semiprime semiprimitive semisimple | |
| Commutative | Primitive field; prime domain; semiprime reduced | |
| Has a minimal left ideal | Prime left primitive right primitive | |
| Left primitive and left artinian |
Relationship Map
The second row of the chart is a strictly decreasing sequence of classes, each properly contained in the previous one. The top row — semiprime, semiprimitive, semisimple — behaves the same way, and the vertical arrows connect them class by class.
The composite *left primitive semiprime* also follows directly from *left primitive prime*, since a prime ring is semiprime. Both routes are used in practice; the module-theoretic one is shorter.
Applications and Industry Use
Applications here means where this structure is used — inside mathematics and in the engineering and computing disciplines that consume it.
Choosing the right generalisation
Wedderburn–Artin describes simple artinian rings. Dropping DCC forces a choice: keep simple and lose all structure, or replace it by left primitive and keep a structure theorem. §11 shows the second choice is the productive one.
Primitive rather than simple
Enveloping algebras of Lie algebras are almost never simple but have many primitive quotients; classification of their primitive ideals is the substitute for a classification of points.
Deciding the class
For a finite-dimensional algebra given by structure constants, makes all six tests equivalent to a radical computation plus a count of simple factors — polynomial time in the dimension.
Simple algebras as primitives
Cyclic algebras and other simple artinian algebras underpin space-time codes and some division-algebra-based schemes; the chart says these are exactly the artinian primitive rings, which is why the finite-dimensional theory suffices there.
Stated honestly: this page is a map, not a tool. Its value is that it prevents wasted effort — knowing that a commutative ring is never interestingly primitive, or that an artinian prime ring is automatically simple, removes whole branches from a search.
Failure Modes and Common Mistakes
- Do not deduce *prime primitive*. and both refute it, and the correct extra hypothesis is the existence of a minimal left ideal or a chain condition .
- Do not deduce *semiprime semiprimitive*. is a domain with .
- Do not read the chart as symmetric in left and right. Only the left primitive entry is side-sensitive, but that one entry is enough to make careless side-swapping unsound.
- Do not assume a primitive ring has a unique simple module. That requires a nonzero socle; without it there may be infinitely many pairwise non-isomorphic faithful simple modules.
Best Practices
- State the chain condition explicitly whenever you reverse an arrow; every reversal on this page depends on one.
- When classifying a ring, settle semiprimeness first — it is the weakest condition and usually the cheapest to test, via nilpotent ideals.
- Record the side of every primitivity claim, and note whether your ring has a minimal one-sided ideal, since that is what removes the ambiguity.
- Use as a sanity filter: if your candidate primitive ring is commutative and not a field, the argument is wrong.
Quick Reference
| Claimed implication | Counterexample | Why it fails |
|---|---|---|
| semiprimitive semisimple | not left artinian | |
| prime left primitive | commutative, not a field | |
| left primitive simple | , infinite | finite-rank ideal is proper and nonzero |
| simple semisimple | no descending chain condition | |
| semiprime semiprimitive | , not nil | |
| left primitive right primitive | Bergman's example | genuine one-sidedness |
Frequently Asked Questions
Why does the chart place primitive between simple and prime rather than somewhere else?
Because the three conditions are increasingly weak constraints on ideals acting on a module. Simple says every nonzero module is faithful; left primitive says at least one simple module is faithful; prime says nonzero ideals cannot multiply to zero, which is what surjective action on a faithful simple module gives you. Each step keeps less information and applies to more rings.
Is there a version of for right artinian rings?
Yes, by applying the left-handed statement to : for a right artinian ring, semisimple, semiprimitive and semiprime coincide, and simple, right primitive, left primitive and prime coincide. What you may not do is mix a left hypothesis with a right conclusion without passing through the opposite ring.
If a ring is prime and has zero radical, must it be primitive?
No. is prime with and is not primitive. Primeness plus semiprimitivity is strictly weaker than primitivity; what closes the gap is a minimal left ideal or a chain condition .
Does mean primitivity is useless for commutative algebra?
It means primitivity adds nothing there: the primitive rings are the fields and the primitive ideals are the maximal ideals. The correct commutative analogue of is the maximal spectrum, and becomes the classical description of the Jacobson radical.
Why is a semisimple ring with several factors not prime?
Because with has nonzero ideals with . This is the step in that forces the number of simple components down to one.
Where does the Density Theorem enter this picture?
It replaces the abstract statement left primitive by a concrete one: is isomorphic to a dense ring of linear transformations on a right vector space over the division ring . Under a left artinian hypothesis this returns , so the Density Theorem contains Wedderburn–Artin for simple artinian rings as a special case.
References
- T. Y. Lam, A First Course in Noncommutative Rings, Graduate Texts in Mathematics 131, Springer-Verlag, 1991, §11, (11.6)–(11.8), pp. 185–187.
- T. Y. Lam, A First Course in Noncommutative Rings, §10 on prime and semiprime rings, and §4 on the Jacobson radical.
- N. Jacobson, Structure of Rings, American Mathematical Society Colloquium Publications 37, revised edition, 1964, Chapters I–IV.
- I. N. Herstein, Noncommutative Rings, Carus Mathematical Monographs 15, Mathematical Association of America, 1968, Chapters 1–2.
- L. H. Rowen, Ring Theory, Volume I, Academic Press, 1988, Chapters 2–3.
AI Suggested Questions
- Give a prime ring with zero Jacobson radical that is neither left nor right primitive, other than a commutative one.
- Does the two-row chart remain valid for rings without identity, and which arrows break?
- How does the chart change if left artinian is weakened to left noetherian?
- Classify the primitive quotients of the enveloping algebra of the two-dimensional nonabelian Lie algebra.
- For a group ring with infinite, which entries of the chart can be decided from properties of alone?
- Show directly that a simple ring with a minimal left ideal is left artinian, and reconcile that with the primitive case.
- What is the analogue of this chart for algebras satisfying a polynomial identity, where Kaplansky's theorem applies?
