Английская Википедия:Absorbing element
Шаблон:Short description In mathematics, an absorbing element (or annihilating element) is a special type of element of a set with respect to a binary operation on that set. The result of combining an absorbing element with any element of the set is the absorbing element itself. In semigroup theory, the absorbing element is called a zero elementШаблон:SfnШаблон:Sfn because there is no risk of confusion with other notions of zero, with the notable exception: under additive notation zero may, quite naturally, denote the neutral element of a monoid. In this article "zero element" and "absorbing element" are synonymous.
Definition
Formally, let Шаблон:Nowrap be a set S with a closed binary operation • on it (known as a magma). A zero element is an element z such that for all s in S, Шаблон:Nowrap. This notion can be refined to the notions of left zero, where one requires only that Шаблон:Nowrap, and right zero, where Шаблон:Nowrap.Шаблон:Sfn
Absorbing elements are particularly interesting for semigroups, especially the multiplicative semigroup of a semiring. In the case of a semiring with 0, the definition of an absorbing element is sometimes relaxed so that it is not required to absorb 0; otherwise, 0 would be the only absorbing element.Шаблон:Sfn
Properties
- If a magma has both a left zero z and a right zero z′, then it has a zero, since Шаблон:Nowrap.
- A magma can have at most one zero element.
Examples
- The most well known example of an absorbing element comes from elementary algebra, where any number multiplied by zero equals zero. Zero is thus an absorbing element.
- The zero of any ring is also an absorbing element. For an element r of a ring R, Шаблон:Nowrap, so Шаблон:Nowrap, as zero is the unique element a for which Шаблон:Nowrap for any r in the ring R. This property holds true also in a rng since multiplicative identity isn't required.
- Floating point arithmetics as defined in IEEE-754 standard contains a special value called Not-a-Number ("NaN"). It is an absorbing element for every operation; i.e., Шаблон:Nowrap, Шаблон:Nowrap, etc.
- The set of binary relations over a set X, together with the composition of relations forms a monoid with zero, where the zero element is the empty relation (empty set).
- The closed interval Шаблон:Nowrap with Шаблон:Nowrap is also a monoid with zero, and the zero element is 0.
- More examples:
| Domain | Operation | Absorber | ||
|---|---|---|---|---|
| real numbers | ⋅ | multiplication | 0 | |
| integers | greatest common divisor | 1 | ||
| n-by-n square matrices | matrix multiplication | matrix of all zeroes | ||
| extended real numbers | minimum/infimum | −∞ | ||
| maximum/supremum | +∞ | |||
| sets | ∩ | intersection | ∅ | empty set |
| subsets of a set M | ∪ | union | M | |
| Boolean logic | ∧ | logical and | ⊥ | falsity |
| ∨ | logical or | ⊤ | truth | |
See also
- Idempotent (ring theory)Шаблон:Sndan element x of a ring such that Шаблон:Nowrap
- Identity element
- Null semigroup
Notes
References
External links
- Absorbing element at PlanetMath
