Primary ideal

In mathematics, specifically commutative algebra, a proper ideal Q of a commutative ring A is said to be primary if whenever xy is an element of Q then x or yn is also an element of Q, for some n > 0. For example, in the ring of integers Z, (pn) is a primary ideal if p is a prime number.

The notion of primary ideals is important in commutative ring theory because every ideal of a Noetherian ring has a primary decomposition, that is, can be written as an intersection of finitely many primary ideals. This result is known as the Lasker–Noether theorem. Consequently,[1] an irreducible ideal of a Noetherian ring is primary.

Various methods of generalizing primary ideals to noncommutative rings exist,[2] but the topic is most often studied for commutative rings. Therefore, the rings in this article are assumed to be commutative rings with identity.

Examples and propertiesEdit

  • The definition can be rephrased in a more symmetric manner: an ideal   is primary if, whenever  , we have   or   or  . (Here   denotes the radical of  .)
  • An ideal Q of R is primary if and only if every zero divisor in R/Q is nilpotent. (Compare this to the case of prime ideals, where P is prime if and only if every zero divisor in R/P is actually zero.)
  • Any prime ideal is primary, and moreover an ideal is prime if and only if it is primary and semiprime.
  • Every primary ideal is primal.[3]
  • If Q is a primary ideal, then the radical of Q is necessarily a prime ideal P, and this ideal is called the associated prime ideal of Q. In this situation, Q is said to be P-primary.
    • On the other hand, an ideal whose radical is prime is not necessarily primary: for example, if  ,  , and  , then   is prime and  , but we have  ,  , and   for all n > 0, so   is not primary. The primary decomposition of   is  ; here   is  -primary and   is  -primary.
      • An ideal whose radical is maximal, however, is primary.
      • Every ideal Q with radical P is contained in a smallest P-primary ideal: all elements a such that ax ∈ Q for some x ∉ P. The smallest P-primary ideal containing Pn is called the nth symbolic power of P.
  • If P is a maximal prime ideal, then any ideal containing a power of P is P-primary. Not all P-primary ideals need be powers of P; for example the ideal (xy2) is P-primary for the ideal P = (xy) in the ring k[xy], but is not a power of P.
  • If A is a Noetherian ring and P a prime ideal, then the kernel of  , the map from A to the localization of A at P, is the intersection of all P-primary ideals.[4]
  • A finite nonempty product of  -primary ideals is  -primary but an infinite product of  -primary ideals may not be  -primary; since for example, in a Noetherian local ring with maximal ideal  ,   (Krull intersection theorem) where each   is  -primary. In fact, in a Noetherian ring, a nonempty product of  -primary ideals   is  -primary if and only if there exists some integer   such that  .[5]


  1. ^ To be precise, one usually uses this fact to prove the theorem.
  2. ^ See the references to Chatters–Hajarnavis, Goldman, Gorton–Heatherly, and Lesieur–Croisot.
  3. ^ For the proof of the second part see the article of Fuchs.
  4. ^ Atiyah–Macdonald, Corollary 10.21
  5. ^ Bourbaki, Ch. IV, § 2, Exercise 3.


  • Atiyah, Michael Francis; Macdonald, I.G. (1969), Introduction to Commutative Algebra, Westview Press, p. 50, ISBN 978-0-201-40751-8
  • Bourbaki, Algèbre commutative.
  • Chatters, A. W.; Hajarnavis, C. R. (1971), "Non-commutative rings with primary decomposition", Quart. J. Math. Oxford Ser. (2), 22: 73–83, doi:10.1093/qmath/22.1.73, ISSN 0033-5606, MR 0286822
  • Goldman, Oscar (1969), "Rings and modules of quotients", J. Algebra, 13: 10–47, doi:10.1016/0021-8693(69)90004-0, ISSN 0021-8693, MR 0245608
  • Gorton, Christine; Heatherly, Henry (2006), "Generalized primary rings and ideals", Math. Pannon., 17 (1): 17–28, ISSN 0865-2090, MR 2215638
  • On primal ideals, Ladislas Fuchs
  • Lesieur, L.; Croisot, R. (1963), Algèbre noethérienne non commutative (in French), Mémor. Sci. Math., Fasc. CLIV. Gauthier-Villars & Cie, Editeur -Imprimeur-Libraire, Paris, p. 119, MR 0155861

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