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Thursday, August 8, 2013

Special Modules over PIDs (12.1.20-22)

Dummit and Foote Abstract Algebra, section 12.1, exercise 20-22:

MathJax TeX Test Page 20. Let R be an integral domain with field of fractions F. Prove dim FRM=rank M for FRM a vector space over F and M any R-module.

21. Let R be a PID. Prove any finitely generated projective R-module is free.

22. Let R be a non-field PID. Prove no finitely generated R-module is injective.

Proof: (20) () We see {fm | fF,mM} spans FRM, so there is a basis {fimi}iI. As well, we see {1mi} is a basis. We claim {mi} is a set of linearly independent elements in M; otherwise, we have rimi=0 implying ri(1mi)=0. () Let {mi} be a set of linearly independent elements. We claim {1mi} are linearly independent over R. We have ri(1mi)=1rimi=1rimi; since the mi are linearly independent, we may assume that if some ri0 then rimi0 and as well rimi is nontorsion; now by 10.4.8 1rimi0 and the 1mi are linearly independent over R. Consequently, collecting the denominators of any linear independence over F leads to an equation of the form 1d(ri(1mi))=0 so that ri(1mi)=0 and now ri=rid=0 for all i and the 1mi are linearly independent over F.

(21) M is projective if and only if it it is a direct summand of a free module only if it is a submodule of a free module, which would imply it has no torsion elements. By the fundamental theorem this eliminates any elementary divisors of M and forces M free.

(22) MRnR/(a1)  ...  R/(ak) is injective if and only if each of its summands is injective by 10.5.4. By Baer's criterion, M is injective over a PID if and only if rM=M for all nonzero rR so necessarily R/(ai) is not injective and we must only prove R is not injective; indeed, for pR a prime we see 1pR. 

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