A new differentiation, shape of the unit ball and perimeter measure Valentino Magnani Indiana University Mathematics Journal 2017 text pdf 10.1512/iumj.2017.66.6007 10.1512/iumj.2017.66.6007 en Indiana Univ. Math. J. 66 (2017) 183 - 204 state-of-the-art mathematics http://iumj.org/access/