On Commuting Self-Adjoint Extensions of Unbounded Operators S. Slinker Indiana University Mathematics Journal 1978 text pdf 10.1512/iumj.1978.27.27041 10.1512/iumj.1978.27.27041 en Indiana Univ. Math. J. 27 (1978) 629 - 636 state-of-the-art mathematics http://iumj.org/access/