@article{MZ_1990__203_3_174118,
     author = {Christian G. Simader},
     title = {An {Elementary} {Proof} of {Harnack's} {Inequality} for {Schr\"odinger} {Operators} and {Related} {Topics.}},
     journal = {Mathematische Zeitschrift},
     pages = {129--152},
     publisher = {mathdoc},
     volume = {203},
     number = {3},
     year = {1990},
     zbl = {0697.35017},
     url = {http://geodesic.mathdoc.fr/item/MZ_1990__203_3_174118/}
}