Executive Summary
Left primitivity is not left-right symmetric. This page identifies the hypothesis that restores the symmetry: the existence of a minimal left ideal. Under it, makes prime, left primitive and right primitive a single condition, and adds a uniqueness statement — every faithful simple left module is isomorphic to the minimal left ideal itself.
The engine is : in a semiprime ring, if is a minimal left ideal then is a minimal right ideal. Consequently the left socle and the right socle of a semiprime ring coincide, and one may speak of the socle . Semiprimeness is essential: a upper triangular matrix ring furnishes the counterexample .
Overview
The socle of a module is the sum of its simple submodules — see The Socle of a Module and of a Ring. Applied to the regular module it gives the left socle , the sum of all minimal left ideals of , and dually the right socle . Both are two-sided ideals. In general they differ.
For semiprime rings they do not, and the reason is a single lemma about individual generators. Once a minimal left ideal is written as , semiprimeness produces enough room to invert a suitably chosen endomorphism of , and that inversion delivers a minimal right ideal . Everything else on this page is a consequence.
The class of rings with nonzero socle is exactly where the classical, finite-rank intuition survives into the infinite-dimensional world. The model is for an infinite-dimensional right -vector space : its socle is the ideal of finite-rank endomorphisms, it is left and right primitive, and it is neither simple nor artinian.
Learning Objectives
- Prove , isolating the exact place where semiprimeness is used.
- Reproduce the counterexample and identify the square-zero ideal responsible.
- Prove , including the uniqueness of the faithful simple module.
- Deduce that left and right socles agree for semiprime rings.
- Show is the ideal of finite-rank maps and that is not artinian.
- Explain why a left primitive ring with several non-isomorphic faithful simple modules must have zero socle.
Definitions
- Minimal left ideal
- A nonzero left ideal with no left ideal strictly between and ; equivalently, is simple as a left -module.
- The left socle: the sum of all minimal left ideals of , or if none exists. It is a two-sided ideal.
- The right socle, defined dually. Equal to the left socle when is semiprime, by .
- The common value, used only when the two socles are known to agree — in particular for semiprime, prime and one-sided primitive rings.
- Semiprime
- No nonzero nilpotent two-sided ideal. The elementwise form used below: .
A minimal left ideal is automatically of the form : if then is a nonzero left ideal inside , so . This is where the identity element is used.
Core Concepts
Minimal left ideals are simple modules sitting inside
A minimal left ideal is a simple left -module that happens to be a submodule of . That double life is what makes the socle so useful: it converts questions about modules into questions about ideals, where multiplication is available.
Two consequences are immediate. First, Schur's Lemma applies: is a division ring for minimal. Second, since , right multiplication by any is a left -module homomorphism , and by simplicity it is either zero or injective. The proof of is exactly the exploitation of that dichotomy.
What semiprimeness supplies
The elementwise characterisation of semiprimeness — implies — says that no element is annihilated by conjugation-like products. Given , it produces with , which is precisely what is needed to make right multiplication by a nonzero endomorphism of the minimal left ideal , hence an isomorphism.
Socle nonzero versus socle zero
For left primitive rings the socle splits the class in two. If , applies: primitivity is two-sided and the faithful simple module is unique. If there are no minimal one-sided ideals at all, the two sides may genuinely diverge, and there can be many non-isomorphic faithful simple modules.
A useful test: a domain that is not a division ring has zero socle. If were a minimal left ideal in a domain then is nonzero, so and for some ; cancelling on the right gives , and in a domain a one-sided inverse is two-sided, so and . Minimality then forces to have no proper nonzero left ideals, i.e. is a division ring.
Key Results
Let be a semiprime ring with identity and . If is a minimal left ideal of , then is a minimal right ideal of .
Note first , since ; hence . It suffices to prove that for every with . Granting this, let be a nonzero right ideal with , and pick . Then , so and ; that is exactly minimality of .
So fix with . Since is semiprime, , so there is with . Define
This is well defined, because for , and it is a homomorphism of left -modules, since it is given by right multiplication. It is nonzero: . As is a simple left module, a nonzero endomorphism is an isomorphism, so has an inverse , again a left -module map.
Now write with and . Left -linearity of gives
as required.
If is semiprime then , and the common ideal is written . In particular this applies to prime rings and to left or right primitive rings, all of which are semiprime by .
Let be a division ring and the ring of upper triangular matrices over . Put . Then is a minimal left ideal, but is not a minimal right ideal: it properly contains the nonzero right ideal . Here is not semiprime, since is a two-sided ideal with .
Let be a ring with identity possessing a minimal left ideal . The following are equivalent:
- is prime;
- is left primitive;
- is right primitive.
If these hold, then also possesses a minimal right ideal , every faithful simple left -module is isomorphic to , and every faithful simple right -module is isomorphic to .
**(2) (1) and (3) (1)** hold for every ring, by .
**(1) (2).** Assume prime. Write with , possible because for any nonzero and minimality forces equality. The module is simple by minimality. It is faithful: if then , and primeness in the form or , together with , gives . So is a faithful simple left module and is left primitive.
Uniqueness. Let be any faithful simple left -module. Since and , we have , so for some . Then is a nonzero submodule of the simple module , hence . The map , , is a surjective homomorphism of left -modules with simple and , hence an isomorphism. Thus .
**(1) (3).** A prime ring is semiprime, so applies to and gives a minimal right ideal . Now run the two paragraphs above in : is a faithful simple right module, is right primitive, and every faithful simple right module is isomorphic to .
A left primitive ring that is not right primitive has zero socle, and in particular has no minimal left and no minimal right ideal. The same applies to a left primitive ring carrying two non-isomorphic faithful simple left modules.
By , a simple ring with a minimal left ideal is already left and right artinian, hence . For left primitive rings this fails completely: with infinite has minimal left and right ideals yet is neither left nor right artinian. Primitivity is genuinely weaker than simplicity, and this is the sharpest illustration.
Proof Techniques and Method
How these proofs work, and which move to reuse.
Right multiplication as an endomorphism
For a left ideal, right multiplication by a fixed element is a left module map. On a minimal left ideal it is therefore either zero or invertible — a dichotomy with no middle ground.
Semiprimeness to avoid the zero case
The elementwise form for is used precisely to guarantee the endomorphism is nonzero. Every use of semiprimeness in this section is of that shape.
Invert and re-associate
Apply the inverse map to a product written so that a left factor can be pulled out. The conclusion is purely a matter of where the brackets are placed.
The uniqueness half of uses a different, equally reusable move: a nonzero homomorphism between simple modules is an isomorphism, so exhibiting any nonzero map finishes the job. Producing that map is the only work, and supplies it.
Worked Example
The socle of
Let be a division ring, a right -vector space, and acting on the left. Fix and let be a projection of onto the line , so and . Put .
Consider , . It is a homomorphism of left -modules and it is surjective, because is simple and . Its kernel is . Indeed , so for every , giving . Conversely suppose . For any we have , say with , whence . So and .
is isomorphic to the simple module , hence is a minimal left ideal of .
So has a minimal left ideal, and is left primitive; by it is also right primitive, is the unique faithful simple left -module up to isomorphism, and is the unique faithful simple right one.
What the socle is
For a rank-one idempotent , the minimal left ideal consists of the endomorphisms vanishing on the fixed hyperplane ; all of them have rank at most one. Conversely any of rank one vanishes on a hyperplane, and choosing to project onto a complementary line gives . Summing over all rank-one idempotents therefore produces every finite-rank map:
A proper nonzero two-sided ideal when is infinite; equal to when is finite.
Not artinian
Take infinite and choose linearly independent in . The left ideals satisfy , since a linear map killing but not exists. So is not left artinian, even though it has minimal left ideals — the contrast with noted above.
Comparison and Classification
| Ring | Minimal left ideal? | Consequence | |
|---|---|---|---|
| yes | all of | simple artinian; unique simple module | |
| , infinite | yes | finite-rank maps, proper and nonzero | left and right primitive, not artinian |
| , of characteristic , non-inner | no | primitive with possibly many faithful simple modules | |
| no | prime, semiprimitive, not primitive | ||
| upper triangular | yes | left and right socles differ | not semiprime; fails |
| Ring primitive on one side only | no | forced by |
| Socles agree | Sides of primitivity agree | Faithful simple module unique | Ring is artinian | |
|---|---|---|---|---|
| Semiprime | yes | no | no | no |
| Prime with a minimal left ideal | yes | yes | yes | no |
| Simple with a minimal left ideal | yes | yes | yes | yes |
| Left primitive, zero socle | yes | no | no | no |
| Left artinian | yes | yes | yes | yes |
What each hypothesis delivers
Relationship Map
- has a minimal left ideal — equivalently
- and is prime
- is left and right primitive
- is the unique faithful simple left module
- has a minimal right ideal
- and is simple
- is left and right artinian
- and is not semiprime
- may fail
- left and right socles may differ — see
- and is prime
Both implications require semiprimeness. The related page Minimal Left Ideals in Semiprime Rings develops the idempotent-theoretic side: in a semiprime ring, a minimal left ideal is generated by an idempotent, which is the structural reason the transfer works.
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 finite-rank ideal
The socle of is the algebraic prototype of the compact operators inside : a canonical proper ideal produced by rank considerations. Much of the ideal theory of operator algebras follows this pattern.
Recognising the regular representation
A minimal left ideal is a copy of a simple module inside the ring itself, so the socle records which irreducible representations are visible in the regular one. For semisimple algebras that is all of them.
Splitting the primitive rings
The socle divides left primitive rings into the tractable case, close to , and the exotic case with zero socle, where pathologies such as one-sided primitivity are confined.
Socle as a certificate
For finite-dimensional algebras the socle is computed alongside the radical and used to certify simplicity of modules and to build composition series in Meataxe-style algorithms.
Honestly stated: this material is internal to algebra. Its practical value is diagnostic — the socle tells you in advance whether the left and right theories of your ring will agree.
Computational Notes
Computational notes cover algorithms, cost and library behaviour rather than manufacturing process.
- For a finite-dimensional algebra over a field given by structure constants, the socle of is computable from the radical: , a nullspace computation once is known.
- Deciding whether a general finitely presented ring has a minimal left ideal is not algorithmic; the word problem already obstructs it. Practical work assumes finite dimension or an explicit model such as .
- For with of countably infinite dimension, elements of the socle are represented by matrices with finitely many nonzero entries, and socle membership is a finite check — this is the standard computational model for the ring.
- In GAP and Magma,
Soclefor algebras and modules is implemented via radical computation; for infinite-dimensional constructions no library support exists and the socle must be described by hand.
Failure Modes and Common Mistakes
- Do not write without knowing the ring is semiprime; otherwise specify or .
- Do not assume every primitive ring has a nonzero socle. Simple domains such as are primitive with socle .
- Do not read as saying primeness implies primitivity in general. The hypothesis has a minimal left ideal is doing all the work; is prime with no minimal left ideal and is not primitive.
- Do not conflate the minimal left ideal with the socle. The socle is the sum of all of them and is two-sided; a single is neither.
Best Practices
- Check semiprimeness first; it is the gateway to every symmetry statement on this page.
- When a minimal left ideal is available, write it as immediately — the generator is what makes the transfer argument computable.
- State which side a socle refers to until the two are proved equal, even in prose.
- Use the uniqueness clause of as a consistency check: if a ring with nonzero socle appears to have two non-isomorphic faithful simple modules, one of the two is not faithful.
Quick Reference
| Observation | Immediate conclusion |
|---|---|
| prime and | left and right primitive; unique faithful simple module each side |
| left primitive, not right primitive | |
| left primitive with two non-isomorphic faithful simple left modules | |
| a domain, not a division ring | |
| simple with a minimal left ideal | , left and right artinian |
Frequently Asked Questions
Why does need semiprimeness rather than primeness?
Because the only thing the proof uses is that implies , which is exactly the elementwise form of semiprimeness. Primeness would be a stronger hypothesis than necessary, and the lemma is applied to semiprime rings that are not prime elsewhere in the theory.
Does mean prime rings are usually primitive?
No. It says prime rings with a minimal left ideal are primitive. Most prime rings have zero socle — , , and every domain that is not a division ring — and for those the theorem says nothing. The hypothesis is restrictive, and its force comes from what it delivers, not from how often it applies.
Can a left primitive ring have two non-isomorphic faithful simple left modules?
Yes, provided its socle is zero. Lam's differential and skew polynomial examples in produce left primitive rings with infinitely many pairwise non-isomorphic faithful simple left modules; forces those rings to have no minimal one-sided ideals.
Is the socle always an ideal?
The left socle is a two-sided ideal of — it is a left ideal by construction, and right multiplication maps a minimal left ideal onto zero or another minimal left ideal, so the sum is stable on the right too. The same holds for the right socle. What is not automatic is that the two coincide.
How does this relate to the Density Theorem?
The Density Theorem describes every left primitive ring as a dense ring of linear transformations on . Rings with nonzero socle are precisely those dense subrings containing some nonzero finite-rank transformation; the socle is then the set of finite-rank elements. Zero socle corresponds to dense subrings avoiding finite rank entirely.
Why is a simple ring with a minimal left ideal automatically artinian, when a primitive one is not?
In a simple ring the socle is a nonzero two-sided ideal, hence all of , so is a sum of simple modules and is semisimple, therefore artinian. In a primitive ring the socle can be a proper ideal, so the argument stops immediately — and shows the gap is real.
References
- T. Y. Lam, A First Course in Noncommutative Rings, Graduate Texts in Mathematics 131, Springer-Verlag, 1991, §11, (11.9)–(11.11), pp. 187–189.
- T. Y. Lam, A First Course in Noncommutative Rings, §3 for (3.10) on simple rings with minimal one-sided ideals, and §10 on prime and semiprime rings.
- N. Jacobson, Structure of Rings, American Mathematical Society Colloquium Publications 37, revised edition, 1964, Chapter IV.
- F. W. Anderson and K. R. Fuller, Rings and Categories of Modules, 2nd edition, Graduate Texts in Mathematics 13, Springer-Verlag, 1992, §9 on socles.
- L. H. Rowen, Ring Theory, Volume I, Academic Press, 1988, Chapter 2.
AI Suggested Questions
- Show that in a semiprime ring every minimal left ideal is generated by an idempotent, and deduce (11.9) from that.
- Describe all dense subrings of containing the finite-rank transformations.
- Give a prime ring with nonzero socle that is not simple and not artinian, other than an endomorphism ring.
- How does the socle of a group algebra of an infinite group behave, and when is it nonzero?
- Prove that the socle of a semiprime ring is a direct sum of its homogeneous components, and identify the components for .
- What is the analogue of (11.11) for rings with a minimal right ideal but no minimal left ideal?
- Compute the left and right socles of for general and explain the asymmetry.
