Idempotent (ring theory)

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In ring theory, a branch of mathematics, an idempotent element or simply idempotent of a ring is an element a such that a2 = a. [1] [lower-alpha 1] That is, the element is idempotent under the ring's multiplication. Inductively then, one can also conclude that a = a2 = a3 = a4 = ... = an for any positive integer n. For example, an idempotent element of a matrix ring is precisely an idempotent matrix.

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For general rings, elements idempotent under multiplication are involved in decompositions of modules, and connected to homological properties of the ring. In Boolean algebra, 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. By the Chinese remainder theorem, this ring factors into the product of rings of integers modulo p, where p is prime. 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 2m idempotents.

We can check this for the integers mod 6, R = Z / 6Z. Since 6 has two prime factors (2 and 3) it should have 22 idempotents.

02 ≡ 0 ≡ 0 (mod 6)
12 ≡ 1 ≡ 1 (mod 6)
22 ≡ 4 ≡ 4 (mod 6)
32 ≡ 9 ≡ 3 (mod 6)
42 ≡ 16 ≡ 4 (mod 6)
52 ≡ 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 3 + 4 ≡ 1 (mod 6), there is a ring decomposition 3Z / 6Z ⊕ 4Z / 6Z. In 3Z / 6Z the multiplicative identity is 3 + 6Z and in 4Z / 6Z the multiplicative identity is 4 + 6Z.

Quotient of polynomial ring

Given a ring R and an element f ∈ R such that f2 ≠ 0, the quotient ring

R / (f2 − f)

has the idempotent f. For example, this could be applied to x ∈ Z[x], or any polynomial f ∈ k[x1, ..., xn].

Idempotents in split-quaternion rings

There is a hyperboloid of idempotents in the split-quaternion ring.[ citation needed ]

Types of ring idempotents

A partial list of important types of idempotents includes:

Any non-trivial idempotent a is a zero divisor (because ab = 0 with neither a nor b being zero, where b = 1 − a). This shows that integral domains and division rings do not have such idempotents. Local rings also do not have such idempotents, but for a different reason. The only idempotent contained in the Jacobson radical of a ring is 0.

Rings characterized by idempotents

Role in decompositions

The idempotents of R have an important connection to decomposition of R-modules. If M is an R-module and E = EndR(M) is its ring of endomorphisms, then A ⊕ B = M if and only if there is a unique idempotent e in E such that A = eM and B = (1 − e)M. Clearly then, M is directly indecomposable if and only if 0 and 1 are the only idempotents in E. [2]

In the case when M = R (assumed unital), the endomorphism ring EndR(R) = R, where each endomorphism arises as left multiplication by a fixed ring element. With this modification of notation, A ⊕ B = R as right modules if and only if there exists a unique idempotent e such that eR = A and (1 − e)R = B. Thus every direct summand of R is generated by an idempotent.

If a is a central idempotent, then the corner ring aRa = Ra is a ring with multiplicative identity a. Just as idempotents determine the direct decompositions of R as a module, the central idempotents of R determine the decompositions of R as a direct sum of rings. If R is the direct sum of the rings R1, ..., Rn, then the identity elements of the rings Ri are central idempotents in R, pairwise orthogonal, and their sum is 1. Conversely, given central idempotents a1, ..., an in R that are pairwise orthogonal and have sum 1, then R is the direct sum of the rings Ra1, ..., Ran. So in particular, every central idempotent a in R gives rise to a decomposition of R as a direct sum of the corner rings aRa and (1 − a)R(1 − a). As a result, a ring R is directly indecomposable as a ring if and only if the identity 1 is centrally primitive.

Working inductively, one can attempt to decompose 1 into a sum of centrally primitive elements. If 1 is centrally primitive, we are done. If not, it is a sum of central orthogonal idempotents, which in turn are primitive or sums of more central idempotents, and so on. The problem that may occur is that this may continue without end, producing an infinite family of central orthogonal idempotents. The condition "R does not contain infinite sets of central orthogonal idempotents" is a type of finiteness condition on the ring. It can be achieved in many ways, such as requiring the ring to be right Noetherian. If a decomposition R = c1R ⊕ c2R ⊕ ... ⊕ cnR exists with each ci a centrally primitive idempotent, then R is a direct sum of the corner rings ciRci, each of which is ring irreducible. [3]

For associative algebras or Jordan algebras over a field, the Peirce decomposition is a decomposition of an algebra as a sum of eigenspaces of commuting idempotent elements.

Relation with involutions

If a is an idempotent of the endomorphism ring EndR(M), then the endomorphism f = 1 − 2a is an R-module involution of M. That is, f is an R-module homomorphism such that f2 is the identity endomorphism of M.

An idempotent element a of R and its associated involution f gives rise to two involutions of the module R, depending on viewing R as a left or right module. If r represents an arbitrary element of R, f can be viewed as a right R-module homomorphism r ↦ fr so that ffr = r, or f can also be viewed as a left R-module homomorphism r ↦ rf, where rff = r.

This process can be reversed if 2 is an invertible element of R: [lower-alpha 2] if b is an involution, then 2−1(1 − b) and 2−1(1 + b) are orthogonal idempotents, corresponding to a and 1 − a. Thus for a ring in which 2 is invertible, the idempotent elements correspond to involutions in a one-to-one manner.

Category of R-modules

Lifting idempotents also has major consequences for the category of R-modules. All idempotents lift modulo I if and only if every R direct summand of R/I has a projective cover as an R-module. [4] Idempotents always lift modulo nil ideals and rings for which R is I-adically complete.

Lifting is most important when I = J(R), the Jacobson radical of R. Yet another characterization of semiperfect rings is that they are semilocal rings whose idempotents lift modulo J(R). [5]

Lattice of idempotents

One may define a partial order on the idempotents of a ring as follows: if a and b are idempotents, we write a ≤ b if and only if ab = ba = a. With respect to this order, 0 is the smallest and 1 the largest idempotent. For orthogonal idempotents a and b, a + b is also idempotent, and we have a ≤ a + b and b ≤ a + b. The atoms of this partial order are precisely the primitive idempotents. [6]

When the above partial order is restricted to the central idempotents of R, a lattice structure, or even a Boolean algebra structure, can be given. For two central idempotents e and f, the complement is given by

¬e = 1 − e,

the meet is given by

e ∧ f = ef.

and the join is given by

e ∨ f = ¬(¬e ∧ ¬f) = e + f − ef

The ordering now becomes simply e ≤ f if and only if eR ⊆ fR, and the join and meet satisfy (e ∨ f)R = eR + fR and (e ∧ f)R = eR ∩ fR = (eR)(fR). It is shown in Goodearl 1991 , p. 99 that if R is von Neumann regular and right self-injective, then the lattice is a complete lattice.

Notes

  1. ↑ Idempotent and nilpotent were introduced by Benjamin Peirce in 1870.
  2. ↑ Rings in which 2 is not invertible are not difficult to find. The element 2 is not invertible in any ring of characteristic 2, which includes Boolean rings.[ clarification needed ]

Citations

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References

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