Lecture Notes on Bayesian Estimation and Classification
《Lecture Notes on Bayesian Estimation and Classification》 · 第 65 页 · Section 3.1「Series and Composition Series」，3.1.2（The Schreier Refinement Theorem）
We can now prove the fundamental theorem on refinements. 3.1.2 (The Schreier Refinement Theorem). Any two $\Omega$-series of an $\Omega$-group possess $\Omega$-isomorphic refinements.