Minimal faces and Schur's Lemma for embeddings into $R^\mathcal{U}$ Scott Atkinson Indiana University Mathematics Journal 2018 text pdf 10.1512/iumj.2018.67.7431 10.1512/iumj.2018.67.7431 en Indiana Univ. Math. J. 67 (2018) 1327 - 1340 state-of-the-art mathematics http://iumj.org/access/