Positive solutions of elliptic boundary value problems without (P.S.) type assumption Song-Ying LiZhaoli LiuZhi-Qiang Wang 35J2035J2535J6035J6535J70elliptic BVPpositive solutionbifurcation diagramvariational methoddescending flow Indiana University Mathematics Journal 2001 text pdf 10.1512/iumj.2001.50.1941 10.1512/iumj.2001.50.1941 en Indiana Univ. Math. J. 50 (2001) 1347 - 1370 state-of-the-art mathematics http://iumj.org/access/