Prove that in "Topics in Algebra 2nd Edition" by I. N. Herstein. Any natural solution that uses
I have to prove that if P is a R-module , P is projective right there is a family in P and morphisms such that for all
where for each for almost all .
We want to prove that is group. I have difficulty proving associativity axiom. The solution reads
Let and . By Theorem 3.4.10 we only need to show
This holds since for all integers a, b, and c by the associative property of the integers. Hence is associative.
Therorem 3.4.10. Let a and b be integers, and let m be a natural number. Then
Let A be a discrete valuation ring, and let be a non-zero element. Compute the integral closure of