On the Order of Zeros of One-Signed Solutions of Elliptic Equations R. Pederson Indiana University Mathematics Journal 1959 text pdf 10.1512/iumj.1959.8.58014 10.1512/iumj.1959.8.58014 en Indiana Univ. Math. J. 8 (1959) 193 - 196 state-of-the-art mathematics http://iumj.org/access/