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 {f⊗m | f∈F,m∈M} spans F⊗RM, so there is a basis {fi⊗mi}i∈I. As well, we see {1⊗mi} is a basis. We claim {mi} is a set of linearly independent elements in M; otherwise, we have ∑rimi=0 implying ∑ri(1⊗mi)=0. (≥) Let {mi} be a set of linearly independent elements. We claim {1⊗mi} are linearly independent over R. We have ∑ri(1⊗mi)=∑1⊗rimi=1⊗∑rimi; since the mi are linearly independent, we may assume that if some ri≠0 then ∑rimi≠0 and as well ∑rimi is nontorsion; now by 10.4.8 1⊗∑rimi≠0 and the 1⊗mi are linearly independent over R. Consequently, collecting the denominators of any linear independence over F leads to an equation of the form 1d(∑ri(1⊗mi))=0 so that ∑ri(1⊗mi)=0 and now ri=rid=0 for all i and the 1⊗mi 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) M≅Rn⊕R/(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 r∈R so necessarily R/(ai) is not injective and we must only prove R is not injective; indeed, for p∈R a prime we see 1∉pR. ◻
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