The square root problem of Kato in one dimension, and first order elliptic systems P. AuscherA. McIntoshA. Nahmod 47F0535J1547A6047B2547E05two-point boundary conditionsfirst order systemsKato square roots Indiana University Mathematics Journal 1997 text pdf 10.1512/iumj.1997.46.1423 10.1512/iumj.1997.46.1423 en Indiana Univ. Math. J. 46 (1997) 659 - 696 state-of-the-art mathematics http://iumj.org/access/