Generalized curvature for certain Radon-like operators of intermediate dimension Philip Gressman Indiana University Mathematics Journal 2019 text pdf 10.1512/iumj.2019.68.7562 10.1512/iumj.2019.68.7562 en Indiana Univ. Math. J. 68 (2019) 201 - 246 state-of-the-art mathematics http://iumj.org/access/