Graded ring

In mathematics, in particular abstract algebra, a graded ring is a ring that is a direct sum of abelian groups $R_i$ such that $R_i R_j \subset R_{i+j}$. The index set is usually the set of nonnegative integers or the set of integers, but can be any monoid. The direct sum decomposition is usually referred to as gradation or grading.

A graded module is defined similarly (see below for the precise definition). It generalizes graded vector spaces. A graded module that is also a graded ring is called a graded algebra. A graded ring could also be viewed as a graded Z-algebra.

The associativity is not important (in fact not used at all) in the definition of a graded ring; hence, the notion applies to a non-associative algebra as well; e.g., one can consider a graded Lie algebra.

First properties

Let

$A = \bigoplus_{n\in \mathbb N_0}A_n = A_0 \oplus A_1 \oplus A_2 \oplus \cdots$

be a graded ring.

• $A_0$ is a subring of A (in particular, the additive identity 0 and the multiplicative identity 1 are homogeneous elements of degree zero.)
• A commutative $\mathbb{N}_0$-graded ring $A=\oplus_0^\infty A_i$ is a Noetherian ring if and only if $A_0$ is Noetherian and A is finitely generated as an algebra over $A_0$.^[1] For such a ring, the generators may be taken to be homogeneous.

Elements of any factor $A_n$ of the decomposition are called homogeneous elements of degree n. An ideal or other subset $\mathfrak{a}$ ⊂ A is homogeneous if, for every element a ∈ $\mathfrak{a}$, when a=a₁+a₂+...+a_n with all a_i homogeneous elements, then all the a_i are in the ideal. For a given a these homogeneous elements are uniquely defined and are called the homogeneous parts of a.

If I is a homogeneous ideal in A, then $A/I$ is also a graded ring, and has decomposition

$A/I = \bigoplus_{n\in \mathbb N}(A_n + I)/I.$

Any (non-graded) ring A can be given a gradation by letting A₀ = A, and A_i = 0 for i > 0. This is called the trivial gradation on A.

Graded module

The corresponding idea in module theory is that of a graded module, namely a left module M over a graded ring A such that also

$M = \bigoplus_{i\in \mathbb{N}_0}M_i ,$

and

$A_iM_j \subseteq M_{i+j}.$

A morphism $f: N \to M$ between graded modules, called a graded morphism, is a morphism of underlying modules that respects grading; i.e., $f(N_i) \subseteq M_i$. A graded submodule is a submodule that is a graded module in own right and such that the set-theoretic inclusion is a morphism of graded modules. Explicitly, a graded module N is a graded submodule of M if and only if it is a submodule of M and satisfies $N_i = N \cap M_i$. The kernel and the image of a morphism of graded modules are graded submodules.

Example: a graded ring is a graded module over itself. An ideal in a graded ring is homogeneous if and only if it is a graded submodule. The annihilator of a graded module is a homogeneous ideal.

Example: to give a graded morphism from a graded ring to a graded ring with the image lying in the center is the same as to give the structure of a graded algebra to the latter ring.

Given a graded module M, the l-twist of $M(l)$ is a graded module defined by $M(l)_n = M_{n+l}$. (cf. Serre's twisting sheaf in algebraic geometry.)

Let M and N be graded modules. If $f: M \to N$ is a morphism of modules, then f is said to have degree d if $f(M_n) \subset N_{n+d}$. An exterior derivative of differential forms in differential geometry is an example of such a morphism having negative degree.

Invariants of graded modules

Given a graded module M over a commutative graded ring A, one can associate the formal power series $P(M, t) \in \mathbb{Z}[\![t]\!]$:

$P(M, t) = \sum \ell(M_n) t^n$

(assuming $\ell(M_n)$ are finite.) It is called the Hilbert–Poincaré series of M.

A graded module is said to be finitely generated if the underlying module is finitely generated. The generators may be taken to be homogeneous (by replacing the generators by their homogeneous parts.)

Suppose A is a polynomial ring $k[x_0, \dots, x_n]$, k a field, and M a finitely generated graded module over it. Then the function $n \mapsto \dim_k M_n$ is called the Hilbert function of M. The function coincides with the integer-valued polynomial for large n called the Hilbert polynomial of M.

Graded algebra

An algebra A over a ring R is a graded algebra if it is graded as a ring.

In the usual case where the ring R is not graded (in particular if R is a field), it is given the trivial grading (every element of R is of grade 0). Thus, R ⊆ A₀ and the A_i are R-modules.

In the case where the ring R is also a graded ring, then one requires that

$A_iR_j \subseteq A_{i+j}$

and

$R_iA_j \subseteq A_{i+j}$.

In other words, we require A to be a left and right graded module over R.

Examples of graded algebras are common in mathematics:

• Polynomial rings. The homogeneous elements of degree n are exactly the homogeneous polynomials of degree n.
• The tensor algebra T^•V of a vector space V. The homogeneous elements of degree n are the tensors of order n, TⁿV.
• The exterior algebra Λ^•V and symmetric algebra S^•V are also graded algebras.
• The cohomology ring H^• in any cohomology theory is also graded, being the direct sum of the Hⁿ.

Graded algebras are much used in commutative algebra and algebraic geometry, homological algebra and algebraic topology. One example is the close relationship between homogeneous polynomials and projective varieties. (cf. homogeneous coordinate ring.)

G-graded rings and algebras

The above definitions have been generalized to gradings ring using any monoid G as an index set. A G-graded ring A is a ring with a direct sum decomposition

$A = \bigoplus_{i\in G}A_i$

such that

$A_i A_j \subseteq A_{i \cdot j}.$

The notion of "graded ring" now becomes the same thing as a N-graded ring, where N is the monoid of non-negative integers under addition. The definitions for graded modules and algebras can also be extended this way replacing the indexing set N with any monoid G.

Remarks:

• If we do not require that the ring have an identity element, semigroups may replace monoids.

Examples:

• A group naturally grades the corresponding group ring; similarly, monoid rings are graded by the corresponding monoid.
• A superalgebra is another term for a Z₂-graded algebra. Examples include Clifford algebras. Here the homogeneous elements are either of degree 0 (even) or 1 (odd).

Anticommutativity

Some graded rings (or algebras) are endowed with an anticommutative structure. This notion requires a homomorphism of the monoid of the gradation into the additive monoid of Z/2Z, the field with two elements. Specifically, a signed monoid consists of a pair (Γ, ε) where Γ is a monoid and ε : Γ → Z/2Z is a homomorphism of additive monoids. An anticommutative Γ-graded ring is a ring A graded with respect to Γ such that:

xy=(-1)^{ε (deg x) ε (deg y)}yx, for all homogeneous elements x and y.

Examples

• An exterior algebra is an example of an anticommutative algebra, graded with respect to the structure (Z_{≥ 0}, ε) where ε: Z → Z/2Z is the quotient map.
• A supercommutative algebra (sometimes called a skew-commutative associative ring) is the same thing as an anticommutative (Z/2Z, ε) -graded algebra, where ε is the identity endomorphism of the additive structure of Z/2Z.

Examples

• The polynomial ring $A = k[t_1, \ldots, t_n]$ is graded by degree: it is a direct sum of $A_i$ consisting of homogeneous polynomials of degree i.
• Let S be the set of all nonzero homogeneous elements in a graded integral domain R. Then the localization of R with respect to S is a Z-graded ring.

See also

• Associated graded ring
• Differential graded algebra
• Filtered algebra, a generalization
• Graded (mathematics)
• Graded category
• Graded Lie algebra
• Graded vector space

References

1. Matsumura 1986, Theorem 13.1

• Lang, Serge (2002), Algebra, Graduate Texts in Mathematics 211 (Revised third ed.), New York: Springer-Verlag, ISBN 978-0-387-95385-4, MR 1878556.
• Bourbaki, N. (1974) Algebra I (Chapters 1-3), ISBN 978-3-540-64243-5, Chapter 3, Section 3.
• H. Matsumura Commutative ring theory. Translated from the Japanese by M. Reid. Second edition. Cambridge Studies in Advanced Mathematics, 8.