Executive Summary
Passing from to is the simplest possible change of rings, and for commutative it has a completely explicit effect on the Jacobson radical. Snapper's Theorem says : the radical of the extension is not built from at all, but from the nilpotent elements of .
Two consequences carry most of the weight. First, is Jacobson semisimple precisely when is reduced — so a local ring with a large radical, such as , acquires a zero radical after one variable is adjoined. Second, on the extension the Jacobson and nilradicals coincide, which is the defining behaviour of a Jacobson (Hilbert) ring and the algebraic content of the Nullstellensatz.
Overview
The governing question of this topic stream is: given rings , what can be said about , and ? No general answer exists, but for specific extensions the answer can be complete. The polynomial extension over a commutative base is the case where it is completely explicit.
Snapper's Theorem. is any nonempty set of commuting indeterminates and is commutative; means the polynomials all of whose coefficients are nilpotent.
Note what is not in the formula. The radical of has vanished. Intuitively, a maximal ideal of contracts to a prime of that need not be maximal, so has far more maximal ideals than the ones sitting over the maximal ideals of — enough of them that their intersection cuts down to the nilpotents.
The noncommutative analogue is genuinely harder, because the nilpotent elements of a noncommutative ring need not form an ideal. The replacement — a nil ideal with — is the subject of Amitsur's Theorem on the Radical of a Polynomial Ring, and the question of whether is the largest nil ideal is open.
Learning Objectives
- State with the commutativity hypothesis and both of its equalities.
- Prove by reduction to the reduced quotient.
- Run the prime-quotient argument that forces the coefficients of into every prime of .
- Explain why does not appear in the answer, using as the test case.
- Deduce : is Jacobson semisimple iff is reduced.
- Contrast the commutative statement with Amitsur's noncommutative theorem.
Definitions
For a commutative ring , denotes the set of nilpotent elements. It is an ideal (commutativity is used here), and a standard theorem of commutative algebra identifies it with the intersection of all prime ideals:
is reduced if .
- The polynomial ring on a set of commuting indeterminates that are central over . Every element involves only finitely many of them, so is the directed union of over finite .
- The set of polynomials all of whose coefficients are nilpotent; equivalently the ideal generated by the nilradical.
- The unit group of .
- Jacobson (Hilbert) ring
- A commutative ring in which every prime ideal is an intersection of maximal ideals; such a ring satisfies rad = Nil, as does every quotient of it.
Throughout this page is commutative with identity and is nonempty. Both hypotheses are used: for the statement is false whenever .
Core Concepts
Units in a commutative polynomial ring
The engine of the proof is the description of for commutative : a polynomial is a unit if and only if and are all nilpotent. One does not need the full statement — only the special case where is a domain, where it says that the units of are the units of , because degrees add.
That special case is applied not to but to each quotient , and the prime ideals then have to be reassembled using .
Why the radical of the base disappears
Suppose is a domain that is not a field, say , whose radical is the maximal ideal . Is ? No: the ideal is proper, since , so it lies in some maximal ideal of — and , since together with would force .
The variable, in other words, supplies inverses for elements of in quotients of , and each such quotient contributes maximal ideals avoiding those elements. Only elements that are nilpotent — hence in every prime, hence unable to be inverted anywhere — survive the intersection.
Key Results
Let be a commutative ring with identity and let be a nonempty set of commuting indeterminates. Then
**Step 1: .** First, if is a reduced commutative ring then so is . It suffices to treat one variable and pass to the directed union, since every polynomial involves finitely many variables. Let have leading coefficient . Because is reduced, , so has leading coefficient and ; iterating, for all , so is not nilpotent. Now apply this to , which is reduced, using : the quotient of by is reduced, whence . Conversely consists of nilpotent elements, since a sum of finitely many commuting nilpotents is nilpotent.
**Step 2: .** A nil ideal is contained in the Jacobson radical, by .
**Step 3: .** Let and fix a variable . Since the radical is an ideal, , hence . Let be any prime ideal and reduce modulo . The ring is a domain, so its units are the units of the domain , i.e. the nonzero constants. Thus the image of is constant, which forces the image of — and hence of — to be . Therefore every lies in . As was arbitrary, gives for every , i.e. .
Combining the three steps closes the cycle of inclusions and gives equality throughout.
With commutative and nonempty, is Jacobson semisimple if and only if is reduced.
By , , and a coefficientwise extension is zero exactly when the coefficient ideal is zero.
For commutative and nonempty, . In particular the contraction of the radical of the extension is a nil ideal of — the commutative shadow of the corresponding statement in Amitsur's Theorem.
Commutativity is used twice: to know that is an ideal, and to know that is the intersection of the primes. Nonemptiness of is used to have a variable available in Step 3; for the assertion reduces to the false claim .
Proof Techniques and Method
How the proof works, and which move transfers to other change-of-rings problems.
Multiply by the variable
From one gets for every . Choosing rather than converts a statement about into a statement about degrees, which is what a domain can see.
Test against every prime quotient
Commutative statements about nilpotence are proved one prime at a time; reassembles the local conclusions. The noncommutative analogue has no such device, which is exactly why Amitsur needed roots of unity instead.
Kill the nilpotents first
Replacing by turns a general commutative ring into a reduced one, and reducedness is inherited by polynomial extensions. This is the standard way to prove a statement modulo the nilradical.
Move 1 generalises: if contains a central element that is a non-zero-divisor and increases degree, the same trick converts radical membership into a coefficient condition. That is precisely how the argument is bootstrapped in the noncommutative setting, with replaced by for a root of unity .
Worked Example
A ring with a big radical whose polynomial ring is semiprimitive
Take , the localisation of at the prime . It is a local domain: . But is a domain, hence reduced, so gives
A ring with nonzero radical whose polynomial extension is Jacobson semisimple.
Direct check that : the element is not a unit of , since is a domain and its units are , which contains no polynomial of degree . This settles the natural question *can while ?* affirmatively.
A ring with nilpotents
Take . Its primes are and , so — computed directly, and no other nonzero element is nilpotent ( cycles through , through ), so . Here as well, since is artinian. Hence
the set of polynomials with all coefficients in . Sanity check via the unit criterion: in because , so is indeed a unit, as membership in the radical demands.
Comparison and Classification
| Ring | |||
|---|---|---|---|
| , a field | |||
| , | |||
| commutative | reduced | noetherian | ||
|---|---|---|---|---|
| is an ideal | yes | no | no | no |
| yes | no | no | no | |
| yes | yes | no | no | |
| yes | yes | yes | no | |
| nilpotent | yes | no | no | yes |
Which hypotheses each conclusion actually needs
Relationship Map
The theorem is the commutative vertex of a small family of change-of-rings statements; the arrows below record which result specialises to which.
- of a polynomial extension — the general question
- commutative
- Snapper:
- semiprimitive iff reduced
- consequence: affine algebras are Jacobson rings
- arbitrary
- Amitsur: with nil
- open: is ? — equivalent to Köthe's conjecture
- a -algebra, scalars enlarged to
- Amitsur: with nil
- commutative
For commutative the first two coincide, and the last inclusion is an equality exactly for Jacobson (Hilbert) rings — the subject of Jacobson Rings, Finitely Generated Algebras and the Nullstellensatz.
Standards and Notation
Standards here covers notation, symbol and markup standards, and reference implementations, rather than material or design codes.
radical I, radical(I) for the ideal-theoretic version of R.ideal(0).radical(), and A.is_reduced() where implementedComputational Notes
Computational notes cover algorithms, cost and library behaviour rather than manufacturing process.
Snapper's Theorem converts the computation of a Jacobson radical — normally an intersection over an infinite family of maximal ideals — into a nilradical computation, which is algorithmic for finitely presented commutative rings.
- For an affine algebra , is , computed by radical-ideal algorithms built on Gröbner bases; Singular, Macaulay2 and Sage all expose this directly.
- Radical membership for a single element is cheaper than the full radical: iff in one extra variable — the Rabinowitsch trick, one Gröbner basis computation.
- Worst-case Gröbner basis complexity is doubly exponential in the number of variables, so the cheap-looking equality is not cheap to evaluate for large presentations.
- For a finite ring given by its multiplication table, can be read off by brute force in polynomial time in , and then gives with no further work.
Failure Modes and Common Mistakes
- Do not conclude that is nilpotent: it is nil, and nilpotent only when is — e.g. when is noetherian.
- Do not assume a maximal ideal of contracts to a maximal ideal of ; it contracts to a prime, and the failure is the whole point.
- Do not read as a statement about being semiprimitive: reducedness and semiprimitivity are independent conditions for commutative rings, as and show in opposite directions.
- Do not silently drop to one variable in the statement: the theorem is about arbitrary, possibly infinite, . Reduction to one variable is legitimate inside the proof only because each polynomial involves finitely many variables.
Historical Notes and Lessons Learned
- 1945Jacobson's radicalThe radical is defined for arbitrary rings via simple modules, making questions like *what is the radical of ?* meaningful outside the artinian world.
- 1950SnapperSnapper, working on completely primary rings, determines the radical of a commutative polynomial ring and identifies it with the extended nilradical.
- 1956AmitsurAmitsur proves the noncommutative analogue: the radical of is for a nil ideal of , and gives the parallel result for the scalar extension .
- 1967EagonEagon gives the streamlined Jacobson-radical route to the Nullstellensatz, in the form Lam follows in –.
- 1972KrempaThe question of whether the nil ideal is the upper nilradical is shown to be equivalent to Köthe's conjecture, which remains open.
The lesson is that a change of rings can simplify an invariant rather than complicate it. Adjoining a variable adds structure to the ring but removes structure from the radical, because it manufactures maximal ideals in directions the base ring could not see.
Quick Reference
| Situation | Answer | Reference |
|---|---|---|
| commutative reduced | (5.2) | |
| commutative | (5.1) | |
| arbitrary | , a nil ideal of | (5.10) |
| with no nonzero nil ideal | (5.10) | |
| commutative noetherian | , nilpotent | (5.1) |
Frequently Asked Questions
Why does not appear in the answer at all?
Because the variable creates new maximal ideals. If is a non-nilpotent element, some prime misses , and in the ideal generated by is proper modulo ; a maximal ideal containing it cannot contain . Only elements lying in every prime — the nilpotents — survive the intersection defining the radical.
Does the theorem hold for one variable only, or for infinitely many?
For any nonempty set , finite or infinite. Nothing in the argument bounds the number of variables: multiplying by a single chosen variable and reducing modulo a prime works verbatim, and every polynomial involves only finitely many variables anyway.
Is nilpotent?
It is nil, always. It is nilpotent precisely when is nilpotent, which holds for noetherian but not in general — take , where is nil of unbounded index.
What replaces this theorem for noncommutative rings?
Amitsur's Theorem: where is a nil ideal of . The difference is that is not identified intrinsically. Whether is always the upper nilradical is equivalent to Köthe's conjecture and is open.
How does the power series ring compare?
It behaves in the opposite way. lies in because is invertible by geometric series, so is the full preimage of and is typically large, whereas is typically small. A polynomial ring has too many maximal ideals; a power series ring has too few.
Does say that is a Jacobson ring?
It says , which is the Jacobson-ring condition applied to the zero ideal only. The full statement — every prime of is an intersection of maximal ideals, whenever is Jacobson — is , and needs the finitely generated algebra machinery.
References
- T. Y. Lam, A First Course in Noncommutative Rings, Graduate Texts in Mathematics 131, Springer-Verlag, 1991, §5, results (5.1)–(5.2) (pp. 70–72).
- E. Snapper, “Completely primary rings. I”, Annals of Mathematics 52 (1950), 666–693.
- S. A. Amitsur, “Radicals of polynomial rings”, Canadian Journal of Mathematics 8 (1956), 355–361.
- I. Kaplansky, Commutative Rings, revised edition, University of Chicago Press, 1974, Chapter 1.
- M. F. Atiyah and I. G. Macdonald, Introduction to Commutative Algebra, Addison-Wesley, 1969, Chapters 1 and 7.
- L. H. Rowen, Ring Theory, Volume I, Academic Press, 1988, Chapter 2.
AI Suggested Questions
- Give a complete proof that a polynomial ring over a reduced commutative ring is reduced, including the infinite-variable case.
- Characterise the units of for a commutative ring and show how that characterisation gives an alternative proof of .
- Exhibit a commutative ring whose nilradical is nil but not nilpotent, and compute the radical of its polynomial ring.
- Compare , and for .
- What is the analogue of Snapper's Theorem for the skew polynomial ring with commutative?
- Explain how is used in the proof that a finitely generated algebra over a field is a Jacobson ring.
- Why does the prime-quotient argument fail for noncommutative rings, and what does Amitsur use in its place?
