In abstract algebra, a multiplicatively closed set (or multiplicative set) is a subset S of a ring R such that the following two conditions hold:[1][2]
- ,
- for all .
In other words, S is closed under taking finite products, including the empty product 1.[3]Equivalently, a multiplicative set is a submonoid of the multiplicative monoid of a ring.
Multiplicative sets are important especially in commutative algebra, where they are used to build localizations of commutative rings.
A subset S of a ring R is called saturated if it is closed under taking divisors: i.e., whenever a product xy is in S, the elements x and y are in S too.
Examples
Examples of multiplicative sets include:
Properties
- An ideal P of a commutative ring R is prime if and only if its complement R \ P is multiplicatively closed.
- A subset S is both saturated and multiplicatively closed if and only if S is the complement of a union of prime ideals.[4] In particular, the complement of a prime ideal is both saturated and multiplicatively closed.
- The intersection of a family of multiplicative sets is a multiplicative set.
- The intersection of a family of saturated sets is saturated.
See also
Notes
- ^ Atiyah and Macdonald, p. 36.
- ^ Lang, p. 107.
- ^ Eisenbud, p. 59.
- ^ Kaplansky, p. 2, Theorem 2.
References
- M. F. Atiyah and I. G. Macdonald, Introduction to commutative algebra, Addison-Wesley, 1969.
- David Eisenbud, Commutative algebra with a view toward algebraic geometry, Springer, 1995.
- Kaplansky, Irving (1974), Commutative rings (Revised ed.), University of Chicago Press, MR 0345945
- Serge Lang, Algebra 3rd ed., Springer, 2002.