@article{IM_1981__63_142798,
     author = {Jean Pierre Bourguignon},
     title = {Les vari\'et\'es de dimension 4 \`a signature non nulle dont la courbure est harmonique sont {d'Einstein.}},
     journal = {Inventiones mathematicae},
     pages = {263--286},
     publisher = {mathdoc},
     volume = {63},
     year = {1981},
     zbl = {0456.53033},
     url = {http://geodesic.mathdoc.fr/item/IM_1981__63_142798/}
}