Asymptotic Expansion of Steady-State Solutions of Symmetric Hyperbolic Linear Differential Equations Robert Lewis Indiana University Mathematics Journal 1958 text pdf 10.1512/iumj.1958.7.57035 10.1512/iumj.1958.7.57035 en Indiana Univ. Math. J. 7 (1958) 593 - 628 state-of-the-art mathematics http://iumj.org/access/