Essential Self Adjointness for the Dirac Operator and Its Square Joseph Wolf Indiana University Mathematics Journal 1973 text pdf 10.1512/iumj.1973.22.22051 10.1512/iumj.1973.22.22051 en Indiana Univ. Math. J. 22 (1973) 611 - 640 state-of-the-art mathematics http://iumj.org/access/