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I am reading "Introduction to Ring Theory" by P. M. Cohn (Springer Undergraduate Mathematics Series)
In Chapter 2: Linear Algebras and Artinian Rings we find the definition of an algebra ... ... but in the chapter on module theory on page 342 of Dummit and Foote we find a different definition ... I cannot see how to reconcile these definitions ...
Cohn's definition of a $$k$$-algebra ($$k$$ is a field) reads as follows:View attachment 3275In Cohn's terms, then, presumably an $$R$$-algebra, where $$R$$ is a commutative ring with identity, would be a mapping $$R \times A \to A$$ denoted by $$( \alpha , r ) \to \alpha r$$ such that L.A.1 to L.A.5 hold with $$\alpha$$ and $$\beta \in R$$ instead of $$k$$.
Now, on page 342 Dummit and Foote define an R-algebra as follows:
"Definition. Let $$R$$ be a commutative ring with identity. An $$R$$-algebra is a ring $$A$$ with identity together with a ring homomorphism $$f \ : \ R \to A $$ mapping $$1_R$$ to $$1_A$$ such that the subring $$f(R)$$ of $$A$$ is contained in the center of $$A$$."
I cannot reconcile these two definitions ... can someone please help?
Peter
In Chapter 2: Linear Algebras and Artinian Rings we find the definition of an algebra ... ... but in the chapter on module theory on page 342 of Dummit and Foote we find a different definition ... I cannot see how to reconcile these definitions ...
Cohn's definition of a $$k$$-algebra ($$k$$ is a field) reads as follows:View attachment 3275In Cohn's terms, then, presumably an $$R$$-algebra, where $$R$$ is a commutative ring with identity, would be a mapping $$R \times A \to A$$ denoted by $$( \alpha , r ) \to \alpha r$$ such that L.A.1 to L.A.5 hold with $$\alpha$$ and $$\beta \in R$$ instead of $$k$$.
Now, on page 342 Dummit and Foote define an R-algebra as follows:
"Definition. Let $$R$$ be a commutative ring with identity. An $$R$$-algebra is a ring $$A$$ with identity together with a ring homomorphism $$f \ : \ R \to A $$ mapping $$1_R$$ to $$1_A$$ such that the subring $$f(R)$$ of $$A$$ is contained in the center of $$A$$."
I cannot reconcile these two definitions ... can someone please help?
Peter