Abstract Algebra/Polynomial rings, irreducibility

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Group rings[edit]

Definition 12.1:

Let be a ring, and let be a group. We can merge them into one object, denoted , called the group ring of and (which is a ring), by taking the fundamental set

Failed to parse (unknown function "\middle"): {\displaystyle \left\{ (r_g)_{g \in G} \middle| r_g \neq 0 \text{ only for finitely many } g \in G \right\}} ,

that is, tuples over of ring elements where only finitely many entries are different from zero, together with addition

and multiplication


It is a straightforward exercise to show that this is, in fact, a ring. The same construction can be carried out with monoids instead of rings; it is completely the same with all definitions carrying over. In this case, we speak of the monoid ring.


  • Exercise 12.1.1: Prove that a group ring and a monoid ring is, in general, a ring.