# Transfer of the GPIT Property in Pullbacks

Authors: David E. Dobbs; Jay Shapiro
Journal: International Journal of Mathematics and Mathematical Sciences
 (2012)

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## Abstract

LetT be a commutative ring, I a prime ideal of T, D a subring of T/I, and R the pullback T×T/ID. Ascent and descent results are given for the transfer of the n-PIT and GPIT (generalized principal ideal theorem) properties between T and R. As a consequence, it follows that if I is a maximal ideal of both T and R, then R satisfies n-PIT (resp., GPIT) if and only if T satisfies n-PIT (resp., GPIT).


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## Body

## 1. Introduction

All rings considered in this paper are commutative, with identity. IfA is a ring, Spec(A) denotes the set of all prime ideals of A, and if P∈Spec(A), then htA(P) denotes the height of P in A. As in [1], if n is a nonnegative integer, then we say that a ring A satisfies the n-PIT property if htA(P)≤n whenever P∈Spec(A) is minimal as a prime ideal of A containing a given n-generated ideal of A. Notice that each ring satisfies 0-PIT and that 1-PIT is the PIT (for “principal ideal theorem") property introduced in [2]. Also as in [1], we say that a ring A satisfies the GPIT (for “generalized principal ideal theorem") property in case A satisfies n-PIT for all n≥0. A well-known fundamental result states that each Noetherian ring satisfies GPIT (cf. [3, Theorem  152]). Moreover, GPIT figures implicitly in a characterization of Noetherian rings [2, Theorem  2.6 and Remark  5.3(a)]. The diversity of rings satisfying PIT [2, Corollaries  3.5, 3.11, and 6.6] and GPIT [1, Corollaries 2.3 and 4.3] has been exhibited by studying the transfer of these properties for various ring-theoretic constructions (cf. [2, Propositions  3.1(a), 4.1 and 6.3, Theorem  4.5 and Section  5], [1, Proposition  2.1, Theorems  3.3 and 4.2]). In particular, in view of the characterizations of Noetherian rings arising as certain pullbacks ([4, Proposition  1.8], [5, Proposition  1, Corollaire  1]), earlier papers developed transfer results for PIT and GPIT in pullbacks D×T/IT in case (T,I) is a quasilocal domain and D is a subring of T/I (see [2, Corollary  3.2(c)], [1, Proposition  2.5(b)]). The main purpose of this note is to extend those results to the context in which T is not necessarily quasilocal or a domain. We thus enlarge the arena of applications in the same spirit in which the general D+M construction studied by Brewer and Rutter [6] extended the classical D+M construction (as in [7]) that had issued from a valuation (in particular, quasilocal) domain T.It is convenient to organize our work so that the assertions regarding transfer of GPIT are consequences of transfer results onn-PIT. Our main descent (resp., ascent) result for these properties is Theorem 2.2 (resp., Theorem 2.5). Technical results dealing with new conditions on prime ideals in pullbacks are isolated in Lemmas 2.1 and 2.4. Our best “if and only if" transfer result appears in Corollary 2.6, with the special case for the D+M construction in Corollary 2.7.In addition to the notation and terminology introduced above, we letMax(A) denote the set of all maximal ideals of a ring A. Any other material is standard, as in [7] or [3].


## 2. Results

Henceforth, we adopt the followingstanding hypotheses: T is a ring, I∈Spec(T), D is a subring of T/I, and R is the pullback T×T/ID. Before giving our main descent result for the n-PIT and GPIT properties, we begin with a lemma that builds on a result of Cahen [5, Proposition  5], that compares the heights of I in T and R. An additional hypothesis is used in Lemma 2.1, since an example of Cahen [5, Exemple  3], shows that the standing hypotheses are not sufficient to imply that htR(I)=htT(I).Lemma 2.1.
LetT, I, D, and R be as in the standing hypotheses. Assume also that if Q∈Spec(T) satisfies htT(Q)≥htT(I), then either Q is comparable to I (with respect to inclusion) or Q+I=T. Then, htR(I)=htT(I).Proof.
According to [5, Proposition  5], htT(I)≤htR(I). Thus, without loss of generality, we may assume that n:=htT(I)<∞. The assertion can be proved by applying the fundamental gluing result on the spectra of pullbacks [4, Theorem  1.4]. We prefer the following somewhat more transparent argument.
Suppose that the assertion fails. Then, we can chooseq∈Spec(R) such that q⊂I and htR(q)≥n. By applying the isomorphism in [4, Corollary  1.5(3)], we see that there exists a (uniquely determined) Q∈Spec(T) such that Q∩R=q and, moreover, that htT(Q)=htR(q)≥n. In view of the hypothesis, there are three cases to consider.
IfQ⊇I, then q=Q∩R⊇I∩R=I, a contradiction. On the other hand, if Q⊂I, then htT(I)≥htT(Q)+1≥n+1, also a contradiction. We handle the remaining case, in which Q+I=T, by an argument that is reminiscent of a proof of Cahen [5, Lemme  5]. In this case, α+i=1, for some α∈Q and i∈I. Then,
(2.1)α=1-i∈Q∩R=q⊂I
and so 1=α+i∈I+I⊆I, the desired contradiction.Theorem 2.2.
LetT, I, D, and R be as in the standing hypotheses. Assume that if Q∈Spec(T) satisfies htT(Q)≥htT(I), then either Q is comparable to I or Q+I=T. Assume also that I∈Max(R). Then,(a)
ifn is a positive integer and T satisfies n-PIT, then R satisfies n-PIT;(b)
ifT satisfies GPIT, then R satisfies GPIT.Proof.
The assertion in (b) follows from that in (a), by universal quantification onn. As for (a), consider an n-generated ideal J=∑i=1nRai of R and let p be minimal among the prime ideals of R that contain J. Our task is to show that htR(p)≤n.
Suppose first thatp=I. Then, every prime of T that is contained in I is actually a prime ideal of R, and so it follows easily that I is minimal as a prime ideal of T containing JT=∑i=1nTai. Since T satisfies n-PIT, htT(I)≤n. Therefore, by Lemma 2.1, htR(p)=htR(I)=htT(I)≤n.
In the remaining case,p≠I. Since I∈Max(R), it follows that I⊈p. An appeal to [4, Corollary  1.5(3)] yields a (uniquely determined) P∈Spec(T) such that P∩R=p. Clearly, I⊈P. Moreover, since P is uniquely determined, it follows easily that P is minimal among primes of T that contain JT. As T satisfies n-PIT, we have that htT(P)≤n. Furthermore, since I⊈p, [4, Theorem  1.4(c)] ensures that Rp=TP. Accordingly
(2.2)ht⁡R(p)=ht⁡Rp(pRp)=ht⁡TP(PTP)=ht⁡T(P)≤n.Recall (cf. [8]) that a ring A is said to be treed in case no maximal ideal of A contains prime ideals of A that are incomparable.Corollary 2.3.
LetT, I, D, and R be as in the standing hypotheses. Assume also that I∈Max(R) and either I∈Max(T) or T is treed. Then,(a)
ifn is a positive integer and T satisfies n-PIT, then R satisfies n-PIT;(b)
ifT satisfies GPIT, then R satisfies GPIT.Proof.
The assumptions ensure that ifQ∈Spec(T), then either Q is comparable to I or Q+I=T. An application of Theorem 2.2 completes the proof.We next isolate a lemma of some independent interest. Notice that the assumption in Lemma2.4 arose naturally in the proof of Corollary 2.3.Lemma 2.4.
LetT, I, D, and R be as in the standing hypotheses. Assume that if Q∈Spec(T), then either Q is comparable to I or Q+I=T. If P∈Spec(R) and P⊆I, then P∈Spec(T).Proof.
We adapt the proof of Lemma2.1. Let P∈Spec(R) such that P⊆I. Since I∈Spec(T), we may assume that P⊂I; in particular, P⊉I. Therefore, by [4, Corollary  1.4(3)], P=Q∩R for some (uniquely determined) Q∈Spec(T). In view of the hypothesis, there are three cases to consider.
The caseQ⊇I is handled as in the proof of Lemma 2.1. On the other hand, if Q⊂I, then Q⊂R, and so P=Q∩R=Q∈Spec(T). Finally, the ostensibly final case, Q+I=T, cannot actually arise, for otherwise, the proof of Lemma 2.1 would show that 1∈I, a contradiction.We next present our main ascent result for then-PIT and GPIT properties.Theorem 2.5.
LetT, I, D, and R be as in the standing hypotheses. Assume also that I∈Max(T). Then,(a)
ifn is a positive integer and R satisfies n-PIT, then T satisfies n-PIT;(b)
ifR satisfies GPIT, then T satisfies GPIT.Proof.
As in the proof of Theorem2.2, it suffices to establish (a). To that end, let P be minimal as a prime ideal of T containing a given n-generated ideal J=∑i=1nTai. We consider two cases.
Suppose first thatP≠I, that is, P⊉I. Choose α∈I∖P. We claim that P is minimal as a prime ideal of T containing the n-generated ideal H:=∑i=1nTaiα. If not, pick Q∈Spec(T) such that H⊆Q⊂P. By the minimality of P, ai∉Q for some i, and so α∈Q since Q is prime. Then, α∈P, a contradiction, thus proving the above claim.
Sinceα∈I⊂R, it is clear that p:=P∩R contains the elements a1α,…,anα. Moreover, it follows from the isomorphism in [4, Corollary  1.5(3)] and the minimality of P that p is minimal among the prime ideals of R containing the n-generated ideal ∑i=1nRaiα. Furthermore, this isomorphism yields that htT(P)=htR(p) and the assumption that R satisfies n-PIT yields that htR(p)≤n. Thus, htT(P)≤n, as desired.
It remains only to consider the caseP=I. Since I∈Max(T), Lemma 2.4 may be applied, with the upshot that htT(I)=htR(I). Suppose that G:=∑i=1nRai and P0∈Spec(R) are such that G⊆P0⊆P=I. Then, by Lemma 2.4 and the minimality of P, we have that J⊆P0∈Spec(T) and P0=P. Hence, P is minimal as a prime ideal of R containing G. Since R satisfies n-PIT, htR(P)≤n. Thus, htT(P)=htT(I)=htR(P)≤n, which completes the proof.Combining Corollary2.3 and Theorem 2.5, we obtain the following sufficient conditions for the GPIT property in R to be equivalent to the GPIT property in T.Corollary 2.6.
LetT, I, D, and R be as in the standing hypotheses. Assume also that I∈Max(R) and I∈Max(T). Then(a)
for any positive integern, R satisfies n-PIT if and only if T satisfies n-PIT;(b)
R satisfies GPIT if and only T satisfies GPIT.We next state a special case of the previous corollary that generalizes [2, Corollary  3.2(c)] and [1, Proposition  2.5(b)] to the general D+M context of [6] in which T need not be quasilocal.Corollary 2.7.
LetB=K+M be a ring, where K is a field and M∈Max(B). Let k be a subfield of K, and put A:=k+M. Then,(a)
for any positive integern, A satisfies n-PIT if and only if B satisfies n-PIT;(b)
A satisfies GPIT if and only B satisfies GPIT.Remark 2.8.
(a) It was stated in Section1 that our results generalize transfer results for PIT and GPIT that had been given in [1, 2] for the case in which (T,I) is quasilocal. In those earlier results, the PIT (resp., GPIT) property for R was shown to be equivalent to the condition that I∈Max(R) and the n-PIT (resp., GPIT) property for T. However, the condition that I∈Max(R) appears as a hypothesis, rather than a conclusion, in Theorem 2.2. Accordingly, we should underscore that, for each n>0, the GPIT property for T does not imply the n-PIT property for R under our standing hypothesis, even if I∈Max(T).
To see this, begin by takingD to be an n-dimensional Noetherian unique factorization domain. Let L denote the quotient field of D. Set T:=L[X]=L+I, with I:=XT, and R:=T×T/ID=T×LD=D+XL[X]. Observe that the standing hypotheses are satisfied. Moreover, I∈Max(T) and, since T is a Noetherian ring, T satisfies GPIT (and, hence, n-PIT). However, R does not satisfy n-PIT.
Indeed, pick a prime elementp of D, so that htD(pD)=1 by the classical principal ideal theorem. We have I⊆pR, essentially since X=p(p-1X). Moreover, pR∈Spec(R), since R/pR≅D/pD. Then, the order-theoretic impact of the fundamental gluing result for pullbacks [4, Theorem  1.4] yields that htR(pR)=n+1, even though pR is an n-generated ideal of R, and so R does not satisfy n-PIT.
(b) Apropos of generalizing the pullback-theoretic results on PIT and GPIT in [1, 2], we do not know whether the standing hypotheses, coupled with the additional assumptions that I∈Max(T) and R satisfies n-PIT with n>0, implies that I∈Max(R). However, we do have the following positive result along these lines for the D+M context. Let B=K+M be a domain, where K is a field and 0≠M∈Max(B). Let D be a subring of K, and put A:=D+M. If A satisfies PIT, then D is a field.
For an indirect proof, suppose that we can pick a nonzero nonunitd∈D. Then, for all α∈M, we have α=d(d-1α)∈d(KM)⊆dM⊆dA, whence M⊆dA. Moreover, M≠dA (the point being that d∉M since M∩D⊆M∩K=0). As d is a nonunit of A, we can choose P minimal among the prime ideals of A containing dA. Then, htA(P)≥htA(M)+1≥2, contrary to A satisfying PIT.
(c) We close by answering a question of the referee that asked for additional examples of a ringT satisfying the hypothesis of Theorem 2.2. Specifically, we show that for each positive integer n≥2, there exists a nontreed non-quasilocal n-dimensional domain T and a nonmaximal nonzero prime ideal I of T such that, whenever Q∈Spec(T) satisfies htT(Q)≥htT(I), then either Q is comparable to I (with respect to inclusion) or Q+I=T.
For the details of the construction, suppose first thatn≥3. Then, a suitable T can be found that possesses exactly n+3 prime ideals. Indeed, consider the (n+3)-element poset S={0=P0,P1,…,I=Pn-1,N=Pn,M1,M=M2}, where the only nontrivial relations are given by Pi<Pj if and only if 0≤i<j≤n; P0<M1<M; and M1<N. It is known (cf. [9, Theorem  2.10]) that there exists a ring B such that Spec(B) is order-isomorphic to S, since S is a finite poset. Then, taking T to be the associated reduced ring of B suffices, for the only prime ideal Q≠I of T that has height at least that of I is N, which is comparable to I. Note that T has exactly two maximal ideals, namely, N and M. The reader is invited to augment the above construction and thus produce an example T with any desired finite number ≥3 of maximal ideals.
Finally, in casen=2, one can produce a suitable T by arguing as above with the 6-element poset S={0,I,N,P,Q,M}, where the only nontrivial relations are given by 0<I<N,0<P<M, and 0<Q<M. The simple verification in this case is left to the reader.
(d) We note that the paper [10] touches on some related matters. Indeed, [10, Theorem  2.7] shows that each ϕ-Noetherian ring satisfies GPIT, while [10, Theorem 2.2] characterizes a ϕ-Noetherian ring as a ϕ-ring R such that ϕ(R) is the pullback of a certain type of diagram.



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