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 $R_{i}R_{j}\subseteq R_{{i+j}}$ . 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

R=\bigoplus _{n\in \mathbb {N} _{0}}R_{n}=R_{0}\oplus R_{1}\oplus R_{2}\oplus \cdots

be a graded ring. Elements of any factor $R_{n}$ of the decomposition are called homogeneous elements of degree n. Every element a of R may be uniquely written as a sum a = a₁ + a₂ + ... + a_n with all a_i homogeneous elements of distinct R_i. These a_i are called the homogeneous components of a.

Some basic properties are:

$R_{0}$ is a subring of R; in particular, the additive identity 0 and the multiplicative identity 1 are homogeneous elements of degree zero.
A commutative $\mathbb {N} _{0}$ -graded ring $R=\bigoplus _{i=0}^{\infty }R_{i}$ is a Noetherian ring if and only if $R_{0}$ is Noetherian and R is finitely generated as an algebra over $R_{0}$ .^[1]

An ideal $I\subseteq R$ is homogeneous if, for every element $a\in I$ , its homogeneous components belong also to $I.$ (Equivalently, they are graded submodules of R; see § Graded module.) The intersections of a homogeneous ideal $I$ with the $R_{i}$ are called the homogeneous parts of $I$ . A homogeneous ideal is the direct sum of its homogeneous parts.

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

R/I=\bigoplus _{n\in \mathbb {N} }(R_{n}+I)/I.

Basic examples

Any (non-graded) ring R can be given a gradation by letting $R_{0}=R$ , and $R_{i}=0$ for i ≠ 0. This is called the trivial gradation on R.
The polynomial ring $R=k[t_{1},\ldots ,t_{n}]$ is graded by degree: it is a direct sum of $R_{i}$ 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.
If I is an ideal in a commutative ring R, then $\oplus _{0}^{{\infty }}I^{n}/I^{{n+1}}$ is a graded ring called the associated graded ring of R along I; geometrically, it is the coordinate ring of the normal cone along the subvariety defined by I.

Graded module

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

M=\bigoplus _{i\in \mathbb {N} _{0}}M_{i},

and

R_{i}M_{j}\subseteq M_{i+j}.

Example: a graded vector space is an example of a graded module over a field (with the field having trivial grading).

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: Given an ideal I in a commutative ring R and an R-module M, $\bigoplus _{n=0}^{\infty }I^{n}M/I^{n+1}M$ is a graded module over the associated graded ring $\oplus _{0}^{{\infty }}I^{n}/I^{{n+1}}$ .

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.

Remark: 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 $M(\ell )$ is a graded module defined by $M(\ell )_{n}=M_{n+\ell }$ . (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 $f(M_{n})\subseteq N_{n+d}$ . An exterior derivative of differential forms in differential geometry is an example of such a morphism having degree 1.

Invariants of graded modules

Given a graded module M over a commutative graded ring R, 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 R 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 degree 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_{i}R_{j}\subseteq A_{i+j}

and

R_{i}A_{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 R is a ring with a direct sum decomposition

R=\bigoplus _{i\in G}R_{i}

such that

R_{i}R_{j}\subseteq R_{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.
An (associative) 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)^{\varepsilon (\deg x)\varepsilon (\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, 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.

Graded monoid

Intuitively, a graded monoid is the subset of a graded ring, $\bigoplus _{n\in \mathbb {N} _{0}}R_{n}$ , generated by the $R_{n}$ 's, without using the additive part. That is, the set of elements of the graded rings is $\bigcup _{n\in \mathbb {N} _{0}}R_{n}$ .

Formally, a graded monoid^[2] is a monoid $(M,\cdot)$ , with a gradation function $\phi :M\to \mathbb {N} _{0}$ such that $\phi (m\cdot m')=\phi (m)+\phi (m')$ . Note that the gradation of $1_{M}$ is necessarily 0. Some authors request furthermore that $\phi (m)\neq 0$ when m is not the identity.

Assuming the gradations of non identity elements are non zero, the number of elements of gradation n is at most ${\frac {g^{n+1}-1}{g-1}}$ where g is the cardinality of a generator G of the monoid. Indeed, each such element is the product of at most n elements of G, and only ${\frac {g^{n+1}-1}{g-1}}$ such product exists. Similarly, the identity element can not be written as the product of two non-identity elements. That is, there is no unit divisor in such a graded monoid.

Power series indexed by a graded monoid

This notions allows to extends the notion of power series ring. Instead of having the indexing family being $\mathbb N$ , the indexing family could be any graded monoid, assuming that the number of elements of degree n is finite, for each integer n.

More formally, let $(K,+_{K},\times _{K})$ be an arbitrary semiring and $(R,\cdot ,\phi )$ a graded monoid. Then $K\langle \langle R\rangle \rangle$ denote the power series with coefficient in K indexed by R. Its elements are functions from R to K. The sum of two elements $s,s'\in K\langle \langle R\rangle \rangle$ is defined point-wise, it is the function sending $m\in R$ to $s(m)+_{K}s'(m)$ . And the product is the function sending defined as the infinite sum $\sum _{p,q\in R,p\cdot q=m}s(p)\times _{K}s'(q)$ . This sum is correctly defined since, for each m, only a finite number of such p and q may exists. Thus, this sum is in fact finite.

Example

In formal language theory, given an alphabet A, the free monoid of words over A can be considered as a graded monoid, where the gradation of a word is its length.

Given a monoid $(M,\cdot)$ , not assumed to be graded, the monoid of finite subsets of M, with product $S\times S'=\{s\cdot s'\mid s\in S,s'\in S'\}$ is a graded monoid, where the graded function is the cardinality of the set.

References

↑ Matsumura 1986, Theorem 13.1
↑ Sakarovitch, Jacques (2009). "Part II: The power of algebra". Elements of automata theory. Translated by Thomas, Reuben. Cambridge: Cambridge University Press. p. 384. ISBN 978-0-521-84425-3. Zbl 1188.68177.

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.

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[1] Matsumura 1986, Theorem 13.1

[2] Sakarovitch, Jacques (2009). "Part II: The power of algebra". Elements of automata theory. Translated by Thomas, Reuben. Cambridge: Cambridge University Press. p. 384. ISBN 978-0-521-84425-3. Zbl 1188.68177.