Monoid ring

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In abstract algebra, a monoid ring is a new ring constructed from some other ring and a monoid.


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

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:

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

The ring R can be embedded in the ring R[G] via the ring homomorphism T : RR[G] defined by

where 1G is the identity element of G.

There also exists a canonical homomorphism going the other way, called the augmentation. It is the map ηR:R[G] → R ,defined by

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


Given a ring R and the (additive) monoid of the natural numbers N (or {xn} viewed multiplicatively), we obtain the ring R[{xn}] =: R[x] of polynomials over R. The Monoid Nn (with the addition) gives the polynomial ring with n variables: R[Nn] =: R[X1, ..., Xn].


  • Lang, Serge (2002). Algebra. Graduate Texts in Mathematics 211 ((Rev. 3rd ed.) ed.). New York: Springer. ISBN 0-387-95385-X. 

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