Some Problems in the Spectral Theory of Stationary Processes on R d Loren Pitt Indiana University Mathematics Journal 1974 text pdf 10.1512/iumj.1974.23.23028 10.1512/iumj.1974.23.23028 en Indiana Univ. Math. J. 23 (1974) 343 - 365 state-of-the-art mathematics http://iumj.org/access/