Monoid ring

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In abstract algebra, a monoid ring is a procedure which constructs a new ring from a given ring and a monoid.

Let R be a ring and G be a monoid. We can look at all the functions φ : G -> R such that the set {g: φ(g) ≠ 0} is finite. We can define addition of such functions to be element-wise additions. We can define multiplication by (φ * ψ)(g) = Σkl=gφ(k)ψ(l). The set of all these functions, together with these two operations, forms a ring, the monoid ring of R over G; it is denoted by R[G]. If G is a group, then it is called the group ring of R over G.

Put less rigorously but more simply, an element of R[G] is a polynomial in G over R, hence the notation. We multiply elements as polynomials, taking the product in G of the "indeterminates" and gathering terms:

(\Sigma_i r_i g_i) \cdot (\Sigma_j s_j h_j) = \Sigma_{i,j} r_i s_j (g_i h_j),

where risj is the product in R and gihj is the product in G.

The ring R can be embedded into the ring R[G] via the ring homomorphism T: R->R[G] defined by

T(r)(1G) = r, T(r)(g) = 0 for g ≠ 1G.

where 1G denotes the identity element in G.

There is also a canonical homomorphism going the other way; the augmentation is the map ηR:R[G] -> R defined by

\sum_{g\in G} r_g g \rightarrow \sum_{g\in G} r_g

The kernel of this homomorphism is called the augmentation ideal and is denoted by JR(G). It is a free R-module generated by the elements 1 - g, for g in G.

[edit] Examples

Given a ring R and the monoid of the non-negative integers, N ({xn} viewed multiplicatively), we obtain the ring R[{xn}] =: R[x] of polynomials over that ring.

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