Linear Partial Differential Systems with an Additional Differential Equation at One Point Avner Friedman Indiana University Mathematics Journal 1958 text pdf 10.1512/iumj.1958.7.57015 10.1512/iumj.1958.7.57015 en Indiana Univ. Math. J. 7 (1958) 173 - 190 state-of-the-art mathematics http://iumj.org/access/