@article{KJM_2010_33_1_a1,
     author = {Vladica Andreji\'c},
     title = {Strong duality principle for four-dimensional {Osserman} manifolds},
     journal = {Kragujevac Journal of Mathematics},
     pages = {17 - 28},
     publisher = {mathdoc},
     volume = {33},
     number = {1},
     year = {2010},
     url = {http://geodesic.mathdoc.fr/item/KJM_2010_33_1_a1/}
}