A weaker notion of integral domain which allows for some zero divisors, but for which one may quotient out the zero divisors to obtain an integral domain.
“Approximate integral domain” is a placeholder name for a concept which may or may not have another name in the mathematics literature. The idea however is that approximate integral domains are to integral domains as local rings are to Heyting fields, and as weak local rings are to weak Heyting fields.
An approximate integral domain is a commutative ring such that:
is nontrivial (); and
The zero divisors form an ideal. (equivalently, the non-cancellative elements form an ideal).
Thus, the quotient of an approximate integral domain by its ideal of zero divisors is an integral domain.
Every approximate integral domain has an equivalence relation , defined as if and only if is a zero divisor. Hence the name “approximate” integral domain. Then integral domains are precisely the approximate integral domains for which implies equality.
In constructive mathematics, similar to the notion of local ring, integral domain, and field, the notion of approximate integral domain bifurcates into multiple distinct notions:
A weak approximate integral domain is an approximate integral domain defined as above.
Recall that a cancellative element in a commutative ring is an element for which both left and right multiplication by is an injection, and zero divisors are precisely the elements in which are not cancellative.
A strict approximate integral domain is a weak approximate integral domain for which additionally the cancellative elements form an anti-ideal:
The quotient ring of a strict approximate integral domain by its anti-ideal of cancellative elements is a Heyting integral domain.
One can define an apartness relation in any strict approximate integral domain: iff is cancellative. Then the approximate integral domain is a Heyting integral domain if and only if this apartness relation is tight.
The addition and multiplication operations on a strict approximate integral domain are strongly extensional with respect to the canonical apartness relation defined by iff is cancellative. In this way a strict approximate integral domain becomes an internal ring object in the category , consisting of sets with apartness relations and maps (strongly extensional functions) between them.
Recall that products in the category of sets with apartness relations is the cartesian product of the underlying sets equipped with the apartness relation defined by iff in or in . Recall also that a function between sets with apartness relations is strongly extensional if implies .
For addition, if , then is cancellative, so or is cancellative since is a strict approximate integral domain, whence . Thus addition is strongly extensional.
For multiplication, if , then is cancellative. Write . Since is a strict approximate integral domain, either is a cancellative element or is a cancellative element. From this we easily conclude is a cancellative element or is, since cancellative elements are closed under multiplicaiton, whence . So multiplication is also strongly extensional.
For a strict approximate integral domain, the ring of fractions obtained by inverting the cancellative elements is a local ring.
The integers are an approximate integral domain which are an integral domain.
The dual integers is an approximate integral domain where the nilpotent infinitesimal is a non-zero zero divisor.
For any prime number and any positive natural number , the prime power local ring is an approximate integral domain, whose ideal of zero divisors is the ideal . The quotient of by its ideal of zero divisors is the finite field , indicating that it is also a weak local ring.
There exist commutative rings which are not approximate integral domains. For example, the integers modulo 6 is not an approximate integral domain, because and are both zero divisors, but is cancellative. When one tries to quotient out the zero divisors, the resulting ring is trivial.
Last revised on August 19, 2024 at 15:00:26. See the history of this page for a list of all contributions to it.