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ArticlePublished 8 Aug 202616 min readBy Kevin Jogin
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Engineering Mathematics Core Change of rings

Radical of Polynomial Rings

For a commutative ring R and any nonempty set T of commuting indeterminates, radR[T]=Nil(R[T])=(NilR)[T]: adjoining a variable destroys the radical of R and leaves only its nilpotents behind.

Page ID
KEVOS-ENG-MATH-NCR-0038
Taxonomy
ENG / ENG-MATH
Collection
noncommutative-rings-core
Source
(5.1)–(5.2), §5 (pp. 70–72)
Reviewed
2026-08-08
Version
1.0.0

Executive Summary

Passing from R to R[T] is the simplest possible change of rings, and for commutative R it has a completely explicit effect on the Jacobson radical. Snapper's Theorem says radR[T]=Nil(R[T])=(NilR)[T]: the radical of the extension is not built from radR at all, but from the nilpotent elements of R.

Two consequences carry most of the weight. First, R[T] is Jacobson semisimple precisely when R is reduced — so a local ring with a large radical, such as (p), 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.

(NilR)[T]radR[T]
ReducedR[T] semiprimitive iff
NoneRole played by radR
1950Snapper

Overview

The governing question of this topic stream is: given rings RS, what can be said about radR, RradS and radS? No general answer exists, but for specific extensions the answer can be complete. The polynomial extension S=R[T] over a commutative base is the case where it is completely explicit.

radR[T]=Nil(R[T])=(NilR)[T]
(5.1)

Snapper's Theorem. T is any nonempty set of commuting indeterminates and R is commutative; (NilR)[T] means the polynomials all of whose coefficients are nilpotent.

Note what is not in the formula. The radical of R has vanished. Intuitively, a maximal ideal of R[T] contracts to a prime of R that need not be maximal, so R[T] has far more maximal ideals than the ones sitting over the maximal ideals of R — 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 N with radR[T]=N[T] — is the subject of Amitsur's Theorem on the Radical of a Polynomial Ring, and the question of whether N is the largest nil ideal is open.

Learning Objectives

  • State (5.1) with the commutativity hypothesis and both of its equalities.
  • Prove Nil(R[T])=(NilR)[T] by reduction to the reduced quotient.
  • Run the prime-quotient argument that forces the coefficients of fradR[T] into every prime of R.
  • Explain why radR does not appear in the answer, using (p) as the test case.
  • Deduce (5.2): R[T] is Jacobson semisimple iff R is reduced.
  • Contrast the commutative statement with Amitsur's noncommutative theorem.

Definitions

DefinitionNilradical and reducedness

For a commutative ring R, NilR 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:

NilR=𝔭SpecR𝔭.
(N)

R is reduced if NilR=0.

R[T]
The polynomial ring on a set T={ti:iI} of commuting indeterminates that are central over R. Every element involves only finitely many of them, so R[T] is the directed union of R[T0] over finite T0T.
(NilR)[T]
The set of polynomials all of whose coefficients are nilpotent; equivalently the ideal (NilR)R[T] generated by the nilradical.
U(R)
The unit group of R.
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 R is commutative with identity and T is nonempty. Both hypotheses are used: for T= the statement is false whenever radRNilR.

Core Concepts

Units in a commutative polynomial ring

The engine of the proof is the description of U(R[t]) for commutative R: a polynomial a0+a1t++antn is a unit if and only if a0U(R) and a1,,an are all nilpotent. One does not need the full statement — only the special case where R is a domain, where it says that the units of R[T] are the units of R, because degrees add.

That special case is applied not to R but to each quotient R/𝔭, and the prime ideals then have to be reassembled using (N).

fradR[T]1tf is a unit of R[T]1tf¯ is a unit of (R/𝔭)[T], hence constantevery coefficient of f lies in 𝔭every coefficient lies in NilR

Why the radical of the base disappears

Suppose R is a domain that is not a field, say R=(p), whose radical is the maximal ideal p(p). Is pradR[t]? No: the ideal (pt1) is proper, since R[t]/(pt1)R[1/p]=0, so it lies in some maximal ideal 𝔪 of R[t] — and p𝔪, since p𝔪 together with pt1𝔪 would force 1𝔪.

The variable, in other words, supplies inverses for elements of R in quotients of R[t], 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

Theorem(5.1)Snapper

Let R be a commutative ring with identity and let T be a nonempty set of commuting indeterminates. Then

radR[T]=Nil(R[T])=(NilR)[T].
Proof

**Step 1: Nil(R[T])=(NilR)[T].** First, if A is a reduced commutative ring then so is A[T]. It suffices to treat one variable and pass to the directed union, since every polynomial involves finitely many variables. Let 0fA[t] have leading coefficient a0. Because A is reduced, a20, so f2 has leading coefficient a2 and degf2=2degf; iterating, f2j0 for all j, so f is not nilpotent. Now apply this to A=R/NilR, which is reduced, using R[T]/(NilR)[T](R/NilR)[T]: the quotient of R[T] by (NilR)[T] is reduced, whence Nil(R[T])(NilR)[T]. Conversely (NilR)[T] consists of nilpotent elements, since a sum of finitely many commuting nilpotents is nilpotent.

**Step 2: Nil(R[T])radR[T].** A nil ideal is contained in the Jacobson radical, by (4.11).

**Step 3: radR[T](NilR)[T].** Let f=iritαiradR[T] and fix a variable tT. Since the radical is an ideal, tfradR[T], hence 1tfU(R[T]). Let 𝔭R be any prime ideal and reduce modulo 𝔭. The ring (R/𝔭)[T] is a domain, so its units are the units of the domain R/𝔭, i.e. the nonzero constants. Thus the image of 1tf is constant, which forces the image of tf — and hence of f — to be 0. Therefore every ri lies in 𝔭. As 𝔭 was arbitrary, (N) gives riNilR for every i, i.e. f(NilR)[T].

Combining the three steps closes the cycle of inclusions and gives equality throughout.

Corollary(5.2)Semiprimitivity criterion

With R commutative and T nonempty, R[T] is Jacobson semisimple if and only if R is reduced.

Proof

By (5.1), radR[T]=(NilR)[T], and a coefficientwise extension is zero exactly when the coefficient ideal is zero.

CorollaryContraction to the base

For R commutative and T nonempty, RradR[T]=NilR. In particular the contraction of the radical of the extension is a nil ideal of R — the commutative shadow of the corresponding statement in Amitsur's Theorem.

RemarkBoth hypotheses are needed

Commutativity is used twice: to know that NilR is an ideal, and to know that NilR is the intersection of the primes. Nonemptiness of T is used to have a variable available in Step 3; for T= the assertion reduces to the false claim radR=NilR.

Proof Techniques and Method

How the proof works, and which move transfers to other change-of-rings problems.

Move 1

Multiply by the variable

From fradS one gets 1gfU(S) for every gS. Choosing g=t rather than g=1 converts a statement about f into a statement about degrees, which is what a domain can see.

Move 2

Test against every prime quotient

Commutative statements about nilpotence are proved one prime at a time; NilR=𝔭 reassembles the local conclusions. The noncommutative analogue has no such device, which is exactly why Amitsur needed roots of unity instead.

Move 3

Kill the nilpotents first

Replacing R by R/NilR 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 SR contains a central element t 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 t replaced by ζt for a root of unity ζ.

Worked Example

A ring with a big radical whose polynomial ring is semiprimitive

Take R=(p), the localisation of at the prime p. It is a local domain: radR=p(p)0. But R is a domain, hence reduced, so (5.2) gives

rad(p)[t]=(Nil(p))[t]=0,
(E.1)

A ring with nonzero radical whose polynomial extension is Jacobson semisimple.

Direct check that pradR[t]: the element 1tp is not a unit of R[t], since R[t] is a domain and its units are U(R), which contains no polynomial of degree 1. This settles the natural question *can radR[t]=0 while radR0?* affirmatively.

A ring with nilpotents

Take R=/12. Its primes are (2) and (3), so NilR=(2)(3){x:xm=0} — computed directly, 62=36=0 and no other nonzero element is nilpotent (2k cycles through 2,4,8,4,, 3k through 3,9,3,), so NilR={0,6}=(6). Here radR=(6) as well, since R is artinian. Hence

rad((/12)[t])=6(/12)[t],
(E.2)

the set of polynomials with all coefficients in {0,6}. Sanity check via the unit criterion: (16t)(1+6t)=136t2=1 in (/12)[t] because 360(mod12), so 16t is indeed a unit, as membership in the radical demands.

Comparison and Classification

The three radicals across commutative examples (T={t})
Ring RNilRradRradR[t]
000
(p)0p(p)0
k[[x]], k a field0(x)0
/12(6)(6)6R[t]
/pn, n2(p)(p)pR[t]
k[x]/(x2)(x)(x)xR[t]
k[x]000
(p)×k[x]/(x2)0×(x)p(p)×(x)0×xR[t]
Which hypotheses each conclusion actually needs
R commutativeTR reducedR noetherian
NilR is an idealyesnonono
Nil(R[T])=(NilR)[T]yesnonono
radR[T]=(NilR)[T]yesyesnono
radR[T]=0yesyesyesno
NilR nilpotentyesnonoyes

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.

  • rad of a polynomial extension — the general question
    • R commutative
      • (5.1) Snapper: radR[T]=(NilR)[T]
      • (5.2) semiprimitive iff reduced
      • consequence: affine algebras are Jacobson rings
    • R arbitrary
      • (5.10) Amitsur: radR[T]=N[T] with N=RradR[T] nil
      • (5.12) open: is N=NilR? — equivalent to Köthe's conjecture
    • R a k-algebra, scalars enlarged to k(T)
      • (5.13) Amitsur: radR(T)=N(T) with N nil
NilRNilRradR

For commutative R 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.

NilradicalNilR (Lam); 𝔑(R), 0 or nil(R) elsewhere
RadicalradR; J(R) in module-theoretic sources
Variable setR[T] with T={ti}iI; R[x1,,xn] in the finite case
Hilbert vs JacobsonLam and Kaplansky say Hilbert ring; Bourbaki and most modern sources say Jacobson ring for the same condition
Macaulay2 / Singularradical I, radical(I) for the ideal-theoretic version of Nil
SageR.ideal(0).radical(), and A.is_reduced() where implemented

Computational 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 A=k[x1,,xm]/I, NilA is I/I, 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: fI iff 1I+(1yf) in one extra variable y — 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 (5.1) is not cheap to evaluate for large presentations.
  • For R a finite ring given by its multiplication table, NilR can be read off by brute force in polynomial time in |R|, and (5.1) then gives radR[T] with no further work.

Failure Modes and Common Mistakes

  • Do not conclude that radR[T] is nilpotent: it is nil, and nilpotent only when NilR is — e.g. when R is noetherian.
  • Do not assume a maximal ideal of R[t] contracts to a maximal ideal of R; it contracts to a prime, and the failure is the whole point.
  • Do not read (5.2) as a statement about R being semiprimitive: reducedness and semiprimitivity are independent conditions for commutative rings, as (p) and k[x]/(x2) show in opposite directions.
  • Do not silently drop to one variable in the statement: the theorem is about arbitrary, possibly infinite, T. 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 R[t]?* 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 R[T] is N[T] for a nil ideal N of R, and gives the parallel result for the scalar extension R(T).
  • 1967EagonEagon gives the streamlined Jacobson-radical route to the Nullstellensatz, in the form Lam follows in (5.3)(5.5).
  • 1972KrempaThe question of whether the nil ideal N 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

Theorem (5.1)R commutative, T: radR[T]=Nil(R[T])=(NilR)[T]
Corollary (5.2)R[T] semiprimitive R reduced
ContractionRradR[T]=NilR
Key inputU(D[T])=U(D) for a domain D
Key inputNilR=𝔭𝔭
Watch out(radR)[T]notradR[T] in general
NoncommutativeUse (5.10): radR[T]=N[T], N nil
Power seriesOpposite behaviour: radR[[x]]=radR+xR[[x]]
Decision table for radR[T]
SituationAnswerReference
R commutative reduced0(5.2)
R commutative(NilR)[T](5.1)
R arbitraryN[T], N a nil ideal of R(5.10)
R with no nonzero nil ideal0(5.10)
R commutative noetherian(NilR)[T], nilpotent(5.1)

Frequently Asked Questions

Why does radR not appear in the answer at all?

Because the variable creates new maximal ideals. If aR is a non-nilpotent element, some prime 𝔭 misses a, and in R[t] the ideal generated by at1 is proper modulo 𝔭; a maximal ideal containing it cannot contain a. 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 T, 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 radR[T] nilpotent?

It is nil, always. It is nilpotent precisely when NilR is nilpotent, which holds for noetherian R but not in general — take R=k[x1,x2,]/(x12,x23,x34,), where NilR is nil of unbounded index.

What replaces this theorem for noncommutative rings?

Amitsur's Theorem: radR[T]=N[T] where N=RradR[T] is a nil ideal of R. The difference is that N is not identified intrinsically. Whether N is always the upper nilradical NilR is equivalent to Köthe's conjecture and is open.

How does the power series ring compare?

It behaves in the opposite way. x lies in radR[[x]] because 1fx is invertible by geometric series, so radR[[x]] is the full preimage of radR and is typically large, whereas radR[t] is typically small. A polynomial ring has too many maximal ideals; a power series ring has too few.

Does (5.1) say that R[T] is a Jacobson ring?

It says radR[T]=NilR[T], which is the Jacobson-ring condition applied to the zero ideal only. The full statement — every prime of R[T] is an intersection of maximal ideals, whenever R is Jacobson — is (5.4), and needs the finitely generated algebra machinery.

References

  1. 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).
  2. E. Snapper, “Completely primary rings. I”, Annals of Mathematics 52 (1950), 666–693.
  3. S. A. Amitsur, “Radicals of polynomial rings”, Canadian Journal of Mathematics 8 (1956), 355–361.
  4. I. Kaplansky, Commutative Rings, revised edition, University of Chicago Press, 1974, Chapter 1.
  5. M. F. Atiyah and I. G. Macdonald, Introduction to Commutative Algebra, Addison-Wesley, 1969, Chapters 1 and 7.
  6. 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 R[t] for a commutative ring R and show how that characterisation gives an alternative proof of (5.1).
  • Exhibit a commutative ring whose nilradical is nil but not nilpotent, and compute the radical of its polynomial ring.
  • Compare radR[t], radR[[t]] and radR[t,t1] for R=(p).
  • What is the analogue of Snapper's Theorem for the skew polynomial ring R[t;σ] with R commutative?
  • Explain how (5.1) 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?
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