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Abelian Groups - László Fuchs [Springer]

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3 Additive <strong>Groups</strong> of Noetherian Rings 683<br />

Noetherian Rings on Free <strong>Groups</strong> The next theorem shows that arbitrarily large<br />

free groups support noetherian domains.<br />

Theorem 3.4 (O’Neill [1]). For every cardinal , the free group on generators<br />

supports a commutative noetherian domain.<br />

Proof. If D n is an integer, then a ring of algebraic integers of degree n is a<br />

Dedekind domain whose additive group is a free group on n generators.<br />

So suppose is infinite, and x . < / are indeterminates. Let P denote<br />

the polynomial ring in these indeterminates with coefficients in Z. The primitive<br />

polynomials (i.e., with 1 as the gcd of coefficients) form a multiplicative semigroup<br />

S in P, and define the ring R as the localization of P at S. Thatis,R consists of<br />

all fractions f =g where f ; g 2 P and g is primitive. Since every polynomial in R is<br />

the product of an integer and primitive polynomials, from the definition it should be<br />

evident that the only ideals of R are nR for integers n 0. This shows that R is a<br />

(commutative) noetherian integral domain.<br />

It remains to show that R C is a free group. That the additive group P C is<br />

free follows from the unique representation of polynomials as sums of different<br />

monomials. We break the proof into several steps.<br />

Step 1. Subrings R and subgroups G . For every ordinal 0

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