Paper 2, Section I, G

Groups, Rings and Modules
Part IB, 2019

Let RR be an integral domain. A module MM over RR is torsion-free if, for any rRr \in R and mM,rm=0m \in M, r m=0 only if r=0r=0 or m=0m=0.

Let MM be a module over RR. Prove that there is a quotient

q:MM0q: M \rightarrow M_{0}

with M0M_{0} torsion-free and with the following property: whenever NN is a torsion-free module and f:MNf: M \rightarrow N is a homomorphism of modules, there is a homomorphism f0:M0Nf_{0}: M_{0} \rightarrow N such that f=f0qf=f_{0} \circ q.