arXiv:1304.5833v1 [] 22 Apr 2013

ON THE EQUALITY OF ORDINARY AND SYMBOLIC POWERS OF
IDEALS
ALINE HOSRY, YOUNGSU KIM AND JAVID VALIDASHTI

Abstract. We consider the following question concerning the equality of ordinary and
symbolic powers of ideals. In a regular local ring, if the ordinary and symbolic powers
of a one-dimensional prime ideal are the same up to its height, then are they the same
for all powers? We provide supporting evidence of a positive answer for classes of prime
ideals defining monomial curves or rings of low multiplicities.

1. Introduction
Let R be a Noetherian local ring of dimension d and P a prime ideal of R. For a
positive integer n, the n-th symbolic power of P , denoted by P (n) , is defined as
P (n) := P n RP ∩ R = {x ∈ R | ∃ s ∈ R \ P, sx ∈ P n }.
Symbolic powers of ideals are central objects in commutative algebra and algebraic geometry for their tight connection to primary decomposition of ideals and the order of
vanishing of polynomials. One readily sees from the definition that P n ⊆ P (n) for all n,
but they are not equal in general. Therefore, one would like to compare the ordinary and
symbolic powers and provide criteria for equality. This problem has long been a subject
of interest, see for instance [2, 8, 11–14, 16, 22]. In this paper, we are interested in criteria
for the equality. In particular, we would like to know if P n = P (n) for all n up to some
value, then they are equal for all n. The following question was posed by Huneke in this
regard.
Question 1.1. Let R be a regular local ring of dimension d and P a prime ideal of height
d − 1. If P n = P (n) for all n ≤ ht P , then is P n = P (n) for all n?
An affirmative answer to Question 1.1 is equivalent to P being generated by a regular
sequence [7]. Furthermore, it is equivalent to showing that if P n = P (n) for all n ≤ d − 1,
then the analytic spread of P is d − 1. This is not known even when P is the defining
ideal of a monomial curve in A4k . Huneke answered Question 1.1 positively in dimension
3, and in dimension 4 if R/P is Gorenstein [15, Corollaries 2.5, 2.6]. One would like
to remove the Gorenstein assumption. There are supporting examples showing that the
Gorenstein property of R/P might follow from P 2 = P (2) . In fact, this is very close to
a conjecture by Vasconcelos which states that if P is syzygetic and R/P and P/P 2 are
Cohen-Macaulay, then R/P is Gorenstein [24]. Note that if P has height d − 1, then
R/P is Cohen-Macaulay, and P/P 2 being Cohen-Macaulay is equivalent to P 2 = P (2) .
Therefore, one is tempted to ask the following question.
Question 1.2. Let R be a regular local ring of dimension d and P a prime ideal of height
d − 1. If P 2 = P (2) , then is R/P Gorenstein?
1

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A. HOSRY, Y. KIM AND J. VALIDASHTI

By Huneke’s result [15, Corollary 2.6], if Question 1.2 has an affirmative answer, then
so does Question 1.1 when dimension of R is 4. The converse of Question 1.2 is true
in dimension 4 by Herzog [9]. Also, Question 1.2 has been answered positively for some
classes of algebras [19], but it is not true in general (see for instance [19, Example 6.1]).
In this paper, we consider the case where P is the defining ideal of a monomial curve
k[[ta1 , . . . , tad ]] and we give an affirmative answer to Questions 1.1 and 1.2 when d = 4 and
{ai } forms an arithmetic sequence. In higher dimensions, if {ai } contains an arithmetic
subsequence of length 5 in which the terms are not necessarily consecutive, we observe
that P 2 6= P (2) , hence we have a positive answer to Questions 1.1 and 1.2. We extend
these results to certain modifications of arithmetic subsequences. We also consider onedimensional prime ideals P of a regular local ring R in general and we show that if R/P
has low multiplicity, then Question 1.1 has a positive answer. We note that if we drop
the height d − 1 assumption on P , then this question does not have a positive answer in
general, due to a counterexample by Guardo, Harbourne and Van Tuyl [10].
2. Monomial Curves
Let a1 , . . . , ad be an increasing sequence of positive integers with gcd(a1 , . . . , ad ) = 1.
Assume that ai ’s generate a numerical semigroup non-redundantly. Consider the monomial curve
A = k[[ta1 , . . . , tad ]] ⊂ k[[t]]
over a field k, with maximal ideal mA := (ta1 , . . . , tad )A. Let R = k[[x1 , . . . , xd ]] be a
formal power series ring with maximal ideal m = (x1 , . . . , xd )R, and let P be the kernel
of the homomorphism
k[[x1 , . . . , xd ]] −→ k[[ta1 , . . . , tad ]]
obtained by mapping xi to tai for all i. Therefore, A is isomorphic to R/P . Note that
P ⊂ m2 because of the non-redundancy assumption on ai ’s. We state the following
well-known properties about monomial curves.
Lemma 2.1. In the above setting,
(1) The ideal ta1 A is a minimal reduction of mA .
(2) The Hilbert-Samuel multiplicity e(mA , A) of A is a1 .
(3) The multiplicity e(mA , A) is at least d, i.e., a1 ≥ d.
For the third property in above, we may assume that k is an infinite field. Then
by [1, Fact (1)], we have e(mA , A) ≥ λ(mA /m2A ), where λ(−) denotes the length, and
observe that λ(mA /m2A ) = d, by the non-redundancy condition on the ai ’s. Note that
the third property also follows from Theorem 3.1. We begin with the following result
that describes the generators of P when d = 4 and the set of exponents {ai } forms an
arithmetic sequence.
Proposition 2.2. Let A be the monomial curve k[[ta , ta+r , ta+2r , ta+3r ]], where a and r
are positive integers that are relatively prime. Regard A as R/P , where R = k[[x, y, z, w]]

ON THE EQUALITY OF ORDINARY AND SYMBOLIC POWERS OF IDEALS

3

and P is the defining ideal of A. Then P is minimally generated by
z 2 − yw, yz − xw, y 2 − xz, xk+r − w k ,

if a = 3k

z 2 − yw, yz − xw, y 2 − xz, xk+r z − w k+1, xk+r y − zw k , xk+r+1 − yw k , if a = 3k + 1
z 2 − yw, yz − xw, y 2 − xz, xk+r+1 − zw k , xk+r y − w k+1 ,

if a = 3k + 2

where k is a positive integer.
Proof. Since the numerical semigroup is non-redundantly generated, a is greater than or
equal to 4 by Lemma 2.1. Thus k ≥ 2 if a = 3k and k ≥ 1 if a = 3k + 1 or 3k + 2. In
each case, let I be the ideal generated by the above-listed elements and m be the maximal
ideal (x, y, z, w) of R. One can directly check that I ⊂ P . For all cases, we will use the
following method to show I = P . First, we show that (P, x) = (I, x). Then it follows
that P = I + x(P : x). But (P : x) = P , since x is not in P . Thus P = I + xP , which
implies P = I, by Nakayama’s Lemma. To show (P, x) = (I, x), let I˜ = (I, x). The short
exact sequence
·y
˜ y) −→ 0
0 −→ R/(I˜ : y) −−−→ R/I˜ −→ R/(I,
˜ = λR (R/(I˜ : y)) + λR(R/(I,
˜ y)). Since R/P is Cohenyields the length equation λR (R/I)
Macaulay and the image of the ideal (x) in R/P is a minimal reduction of m/P by
Lemma 2.1, we have
˜
a = e(m, R/P ) = λR (R/(P, x)) ≤ λR (R/I).
Thus, it is enough to show
˜ y)) ≤ a.
λR (R/(I˜ : y)) + λR (R/(I,
˜ y) = (x, y, z 2 , w k ) and the ideal
If a = 3k, then I˜ = (x, z 2 − yw, yz, y 2, w k ). Therefore, (I,
I˜ : y contains the ideal (x, y, z, w k ). Thus,
˜ y)) ≤ λR (R/(x, y, z, w k )) + λR (R/(x, y, z 2 , w k )) ≤ k + 2k = a.
λR (R/(I˜ : y)) + λR (R/(I,
If a = 3k + 1, then I˜ = (x, z 2 − yw, yz, y 2, w k+1, zw k , yw k ). Hence, (x, y, z, w k ) ⊂ I˜ : y
˜ y) = (x, y, z 2 , zw k , w k+1). Note that λR (R/(x, y, z 2 , zw k , w k+1)) = 2k + 1 and
and (I,
λR (R/(x, y, z, w k )) = k. Thus,
˜ y)) ≤ k + (2k + 1) = a.
λR (R/(I˜ : y)) + λR (R/(I,
If a = 3k + 2, then I˜ = (x, z 2 − yw, yz, y 2, zw k , w k+1). Therefore, (x, y, z, w k+1) ⊂ I˜ : y
˜ y) = (x, y, z 2 , zw k , w k+1). Similar to the previous case, λR (R/(x, y, z 2 , zw k , w k+1))
and (I,
is 2k + 1 and λR (R/(x, y, z, w k+1)) = k + 1. Hence we obtain
˜ y)) ≤ (k + 1) + (2k + 1) = a.
λR (R/(I˜ : y)) + λR (R/(I,
To show that P is minimally generated by the listed elements in each case, we can compute
µ(P ) = λR (P/mP ). In fact, if we let R̄ = R/xR, then
µ(P R̄) = λR (P R̄/mP R̄) = λR (P + (x)/mP + (x)) = λR (P/mP + P ∩ (x)).
But P ∩ (x) = x(P : x) = xP ⊂ mP . Thus µ(P R̄) = µ(P ). Therefore, to compute the
minimal number of generators in each case, we can go modulo (x) first. If a = 3k, we
will show in Theorem 2.3 that P 2 6= P (2) , hence P is not a complete intersection ideal.

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A. HOSRY, Y. KIM AND J. VALIDASHTI

Thus it cannot have fewer number of generators than 4. If a = 3k + 2, we will show
in Theorem 2.3 that P is generated by the 4 by 4 Pfaffians of a 5 by 5 skew-symmetric
matrix. Hence by Buchsbaum-Eisenbud structure theorem for height 3 Gorenstein ideals
in [6], P is minimally generated by the listed elements in this case. Thus, we only need to
deal with the case a = 3k + 1, where one can check directly that the ideal P R̄ is minimally
generated by z 2 − yw, yz, y 2 , w k+1, zw k , yw k .

Theorem 2.3. Let A be the monomial curve A = k[[ta , ta+r , ta+2r , ta+3r ]], where a and r
are positive integers that are relatively prime. Regard A as R/P , where R = k[[x, y, z, w]]
and P is the defining ideal of A.
(1) If a = 3k or 3k + 1, then R/P is not Gorenstein and P 2 6= P (2) .
(2) If a = 3k + 2, then R/P is Gorenstein, P 2 = P (2) and P 3 6= P (3) .
Proof. If a = 3k, then one can see that P contains the 2 by 2 minors of

x
y
z
.
z
w
M = y
k−1
k+r
k+r−1
zw
x
yx


Let D = det(M). Note that D 6∈ P 2, since D is not in P 2 modulo (x, y). We will show
that D ∈ P (2) . We have det(adj(M)) · D = D 3 , where adj(M) is the adjoint matrix of
M. Note that D 6= 0, for example it is not zero modulo (x, y). Thus D 2 = det(adj(M)).
But det(adj(M)) ∈ P 3, since the entries of adj(M) are in P . Hence D 2 ∈ P 3 . Therefore,
the image of D 2 in the associated graded ring GP := grP RP (RP ) is zero. Note that GP
is a domain as RP is a regular local ring. Hence the image of D is zero in GP , which
shows that the image of D in the localization RP is in P 2 RP , i.e., D ∈ P (2) . One could
also directly show that w · det(M) ∈ P 2 , hence det(M) ∈ P (2) , as w is not in P . Now by
Herzog’s theorem [9, Satz 2.8], we conclude that R/P is not Gorenstein. We note that
since in Proposition 2.2 we have shown that P is minimally generated by 4 elements, we
could also use Buchsbaum-Eisenbud structure theorem for height 3 Gorenstein ideals in
[6], or Bresinsky’s result in [3] which states that if a monomial curve in dimension 4 is
Gorenstein, then P is minimally generated by 3 or 5 elements.
If a = 3k + 1, then P contains the 2 by 2 minors of

x
y
z
z
w .
M = y
k−1
k
k+r
zw
w x


With a similar argument as in the previous case, one can show that det(M) ∈ P (2) \ P 2.
Thus by Herzog’s result, R/P is not Gorenstein.

ON THE EQUALITY OF ORDINARY AND SYMBOLIC POWERS OF IDEALS

If a = 3k + 2, then by Proposition 2.2, one can see
Pfaffians of

0
−w k
0
k
k+r
 w
0
x

k+r
0
M =
 0 −x
 −x
−y
−z
−y
−z
−w

5

that P is generated by the 4 by 4

x y
y z 

z w 
.
0 0 
0 0

Thus, by Buchsbaum-Eisenbud structure theorem for height 3 Gorenstein ideals in [6],
we obtain that R/P is Gorenstein and P is minimally generated by the 5 listed elements
in Proposition 2.2. Hence, P 2 = P (2) by Herzog’s result [9, Satz 2.8], and P 3 6= P (3) by
Huneke’s result [15, Corollary 2.6], as P is not a complete intersection ideal.

Corollary 2.4. Question 1.1 and Question 1.2 have affirmative answers for monomial
curves as in Theorem 2.3.
Now we consider monomial curves in higher dimensions.
Theorem 2.5. Let A be the monomial curve k[[ta1 , . . . , tad ]]. Consider A as R/P , where
R = k[[x1 , . . . , xd ]] and P is the defining ideal of A. If {ai } has an arithmetic subsequence
of length 5, whose terms are not necessarily consecutive, then P 2 6= P (2) .
Proof. If {ai } has an arithmetic subsequence {b1 , . . . , b5 } of length 5, without loss of
generality we may assume that x1 , . . . , x5 correspond to tb1 , . . . , tb5 . Then, one can see
that P contains the 2 by 2 minors of


x1 x2 x3
M =  x2 x3 x4  .
x3 x4 x5
We observe that det(M) 6∈ P 2, since det(M) is a polynomial of degree 3 and the generators
of P 2 have degree at least 4 as P ⊂ m2 . Also note that det(M) 6= 0, for example it is not
zero modulo (x2 , x3 ). Thus, by a similar argument as in the proof of Theorem 2.3, one
can show that det(M) ∈ P (2) .

Corollary 2.6. Question 1.1 and Question 1.2 have positive answers for monomial curves
as in Theorem 2.5.
Using a result of Morales [21, Lemma 3.2], we can extend Theorems 2.3 and 2.5 to a
larger class of monomial curves. As before, let A be the monomial curve k[[ta1 , . . . , tad ]].
In the following we will not assume any particular order on the ai ’s. Write A as R/P ,
where R is k[[x1 , . . . , xd ]] and P is the defining ideal of A. For a positive integer c,
relatively prime to a1 , let Ã be the modified monomial curve k[[ta1 , tca2 , . . . , tcad ]]. Note
that a1 , ca2 , . . . , cad non-redundantly generate their numerical semigroup too. Write Ã as
R̃/P̃ , where R̃ denotes k[[x1 , . . . , xd ]] and P̃ is the defining ideal of Ã. Consider R̃ as an
R-module via the map φ : R −→ R̃ that sends x1 to xc1 and fixes xi for all i 6= 1. For a
polynomial f (x1 , . . . , xd ) ∈ R, let f˜ be the polynomial f (xc1 , . . . , xd ).
Lemma 2.7.(Morales) R̃ is a faithfully flat extension of R. Moreover, P R̃ ∩ R = P and
P̃ = P R̃. In fact, f ∈ P if and only if f˜ ∈ P̃ , and if {gi } is a minimal generating set for

6

A. HOSRY, Y. KIM AND J. VALIDASHTI

P , then {g˜i } is a minimal generating set for P̃ . In addition, for all positive integers k,
f ∈ P k if and only if f˜ ∈ P̃ k , and f ∈ P (k) if and only if f˜ ∈ P̃ (k) , i.e., P̃ k ∩ R = P k and
P̃ (k) ∩ R = P (k) .
Using Lemma 2.7, we obtain the following extension of Theorems 2.3 and 2.5.
Corollary 2.8. If Question 1.1 has an affirmative answer for a monomial curve A, then
it also has an affirmative answer for the monomial curve Ã. In particular, Question
1.1 has an affirmative answer for successive modifications of the monomial curves as in
Theorems 2.3 and 2.5 in the sense of Morales.
Proof. If P̃ n = P̃ (n) for all positive integers n ≤ d − 1, then by Lemma 2.7, we obtain
that P n = P (n) for all n ≤ d − 1. Thus, by hypothesis, P is a complete intersection and
hence, by Lemma 2.7, we obtain that P̃ is a complete intersection.


3. Low Multiplicities
Let R be a regular local ring with maximal ideal m and of dimension d. Let P be a
prime ideal of height d − 1. We will show that Question 1.1 has an affirmative answer
when the Hilbert-Samuel multiplicity e(R/P ) is sufficiently small.
Theorem 3.1. Let R be a regular local ring with maximal ideal m and of dimension d.
Assume P is a prime ideal of height d − 1 such that P ⊂ m2 . Then P n 6= P (n) for a
positive integer n, if
d−2
Y
2n + r
.
e(R/P ) <
n+r
r=0
Proof. We may assume the residue field of R is infinite, see for instance [17, Lemma 8.4.2].
Thus, as R/P has dimension one, there exists x ∈ R whose image in R/P is a minimal
reduction of m/P . Note that x cannot be in m2 by Nakayama’s Lemma, hence R/(x)
is regular. Recall that in a regular local ring S with maximal ideal n and of dimension
k, λS (S/nn ) = n+k−1
for all positive integers n. Therefore, since P n ⊂ m2n , we have
k

λR (R/(P n , x)) ≥ λR (R/(m2n , x)) = 2n+d−2
. On the other hand, since R/P is a oned−1
dimensional Cohen-Macaulay ring, using the associativity formula for multiplicities, we
obtain
λR (R/(P (n) , x)) = e((x), R/P (n) )
= λRP (RP /P n RP ) · e((x), R/P )
= n+d−2
· e(R/P ).
d−1


2n+d−2
The multiplicity bound in the statement is equivalent to n+d−2
·
e(R/P
)
<
.
d−1
d−1
(n)
n
n
(n)
Therefore, λR (R/(P , x)) < λR (R/(P , x)). Thus, P and P cannot be the same. 
One can easily observe that the multiplicity bound in Theorem 3.1 is increasing with
respect to n. Thus, letting n = d − 1, we obtain the largest bound that guarantees
P d−1 6= P (d−1) . Therefore, we have the following corollary.

ON THE EQUALITY OF ORDINARY AND SYMBOLIC POWERS OF IDEALS

7

Corollary 3.2. Under the assumptions of Theorem 3.1, Question 1.1 has a positive answer provided
d−2
Y 2d + r − 2
e(R/P ) <
.
d
+
r
−
1
r=0
Note that the multiplicity bound in Corollary 3.2 grows at least exponentially with
respect to d, since each term of the product is greater than 32 . The next corollary is an
application of Theorem 3.1 in the case of monomial curves in embedding dimension 4.
Corollary 3.3. Let A = k[[ta1 , ta2 , ta3 , ta4 ]]. Consider A as R/P , where R = k[[x, y, z, w]]
and P is the defining ideal of A. If a1 = 4 or 5, then P 3 6= P (3) . Therefore, Question 1.1
has a positive answer in this case.
Proof. Apply Theorem 3.1 for n = 3 and d = 4. On the one hand e(R/P ) = a1 ≤ 5, and
on the other hand the multiplicity bound reduces to 5.6. Hence P 3 6= P (3) .

We remark that, by Corollary 2.8, Question 1.1 has an affirmative answer for successive
modifications of the monomial curves as in Corollary 3.3 in the sense of Morales.
4. Remarks
We end this paper with some remarks and observations on equality of the ordinary and
symbolic powers of ideals.
Remark 4.1. The multiplicity bound in Theorem 3.1 approaches 2d−1 as n tends to
infinity. Thus, if e(R/P ) < 2d−1 , then P n 6= P (n) for n large. Hence, if Question 1.1
has a positive answer and P n = P (n) for all n ≤ d − 1, then e(R/P ) ≥ 2d−1 . This is
consistent with the conclusion of Question 1.1, that P is a complete intersection. To
see this, suppose P is generated by a regular sequence a1 , . . . , ad−1 and x is a minimal
reduction of m/P in R/P . Then, by [20, Theorem 14.9], we have
e(m, R/P ) = λR (R/(P, x)) = λR (R/(a1 , . . . , ad )) ≥

d
Y

ordm (ai ) ≥ 2d−1 ,

i=1

where ad = x. Note that ordm (x) = 1 and ordm (ai ) ≥ 2 for i = 1, . . . , d − 1, as we are
assuming P ⊂ m2 .
Remark 4.2. We know that if P n = P (n) for n large, then P is a complete intersection [7].
The conclusion is also true if P n = P (n) for infinitely many n, see for instance Brodmann’s
result on stability of associated primes of R/P n in [4]. This can also be obtained using
superficial elements, at least when R has infinite residue field and P has positive grade.
If P n = P (n) for infinitely many n, then one can show that P n = P (n) for n large. To
see this, let x ∈ P be a superficial element, in the sense that P n+1 : x = P n for n large,
see [17, Proposition 8.5.7]. Hence, if there exists an element b ∈ P (n) \ P n , then we have
xb ∈ P (n+1) \ P n+1 for n large.
Remark 4.3. If P n = P (n) for n large, then the analytic spread of P is at most d − 1
[5]. We note that this can also be seen via ε-multiplicity for one-dimensional primes. For

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A. HOSRY, Y. KIM AND J. VALIDASHTI

a prime ideal P of height d − 1, we have
H0m (R/P n ) = P (n) /P n ,
where the left hand side is the zero-th local cohomology of R/P n with support in m.
Thus, if P n = P (n) for n large, then ε-multiplicity of P is zero, where
ε(P ) = lim sup
n

d!
· λR (H0m (R/P n )).
nd

Hence, by [18, Theorem 4.7] or [23, Theorem 4.2], the analytic spread of P is at most d−1.

Acknowledgements
This research was initiated at the MRC workshop on commutative algebra at Snowbird,
Utah, in the summer of 2010. We would like to thank AMS, NSF and MRC for providing
funding and a stimulating research environment. We are also thankful to the organizers
of the workshop, David Eisenbud, Craig Huneke, Mircea Mustata and Claudia Polini for
many helpful ideas and suggestions regarding this work.
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Department of Mathematics and Statistics, Notre Dame University-Louaize, P.O. Box: 72,
Zouk Mikael, Zouk Mosbeh, Lebanon
E-mail address: ahosry@ndu.edu.lb
Department of Mathematics, Purdue University, West Lafayette, IN 47907
E-mail address: kim455@purdue.edu
Department of Mathematics, University of Illinois at Urbana-Champaign, IL 61801
E-mail address: jvalidas@illinois.edu

