Convolution, subordination and characterization problems in noncommutative probability WIktor EjsmontUwe FranzKamil Szpojankowski Indiana University Mathematics Journal 2017 text pdf 10.1512/iumj.2017.66.5971 10.1512/iumj.2017.66.5971 en Indiana Univ. Math. J. 66 (2017) 237 - 257 state-of-the-art mathematics http://iumj.org/access/