In
ring theory
In algebra, ring theory is the study of rings—algebraic structures in which addition and multiplication are defined and have similar properties to those operations defined for the integers. Ring theory studies the structure of rings, their r ...
, a branch of
abstract algebra
In mathematics, more specifically algebra, abstract algebra or modern algebra is the study of algebraic structures. Algebraic structures include group (mathematics), groups, ring (mathematics), rings, field (mathematics), fields, module (mathe ...
, an idempotent element or simply idempotent of a
ring is an element ''a'' such that . That is, the element is
idempotent
Idempotence (, ) is the property of certain operations in mathematics and computer science whereby they can be applied multiple times without changing the result beyond the initial application. The concept of idempotence arises in a number of pl ...
under the ring's multiplication.
Inductively then, one can also conclude that for any positive
integer
An integer is the number zero (), a positive natural number (, , , etc.) or a negative integer with a minus sign ( −1, −2, −3, etc.). The negative numbers are the additive inverses of the corresponding positive numbers. In the languag ...
''n''. For example, an idempotent element of a
matrix ring is precisely an
idempotent matrix
In linear algebra, an idempotent matrix is a matrix which, when multiplied by itself, yields itself. That is, the matrix A is idempotent if and only if A^2 = A. For this product A^2 to be defined, A must necessarily be a square matrix. Viewed thi ...
.
For general rings, elements idempotent under multiplication are involved in decompositions of
modules, and connected to
homological properties of the ring. In
Boolean algebra
In mathematics and mathematical logic, Boolean algebra is a branch of algebra. It differs from elementary algebra in two ways. First, the values of the variables are the truth values ''true'' and ''false'', usually denoted 1 and 0, whereas i ...
, the main objects of study are rings in which all elements are idempotent under both addition and multiplication.
Examples
Quotients of Z
One may consider the
ring of integers modulo ''n'' where ''n'' is
squarefree
In mathematics, a square-free integer (or squarefree integer) is an integer which is divisible by no square number other than 1. That is, its prime factorization has exactly one factor for each prime that appears in it. For example, is square-fr ...
. By the
Chinese remainder theorem, this ring factors into the
product of rings
In mathematics, a product of rings or direct product of rings is a ring that is formed by the Cartesian product of the underlying sets of several rings (possibly an infinity), equipped with componentwise operations. It is a direct product in th ...
of integers modulo ''p'' where ''p'' is
prime
A prime number (or a prime) is a natural number greater than 1 that is not a product of two smaller natural numbers. A natural number greater than 1 that is not prime is called a composite number. For example, 5 is prime because the only ways ...
. Now each of these factors is a
field, so it is clear that the factors' only idempotents will be 0 and 1. That is, each factor has two idempotents. So if there are ''m'' factors, there will be 2
''m'' idempotents.
We can check this for the integers mod 6, . Since 6 has two prime factors (2 and 3) it should have 2
2 idempotents.
: 0
2 ≡ 0 ≡ 0 (mod 6)
: 1
2 ≡ 1 ≡ 1 (mod 6)
: 2
2 ≡ 4 ≡ 4 (mod 6)
: 3
2 ≡ 9 ≡ 3 (mod 6)
: 4
2 ≡ 16 ≡ 4 (mod 6)
: 5
2 ≡ 25 ≡ 1 (mod 6)
From these computations, 0, 1, 3, and 4 are idempotents of this ring, while 2 and 5 are not. This also demonstrates the decomposition properties described below: because , there is a ring decomposition . In 3Z/6Z the identity is 3+6Z and in 4Z/6Z the identity is 4+6Z.
Quotient of polynomial ring
Given a ring
and an element
such that
, then the
quotient ring
In ring theory, a branch of abstract algebra, a quotient ring, also known as factor ring, difference ring or residue class ring, is a construction quite similar to the quotient group in group theory and to the quotient space in linear algebra. ...
:
has the idempotent
. For example, this could be applied to