@article{AM_1999_150_3_a4,
     author = {Neil Robertson and P.D. Seymour and Robin Thomas},
     title = {Permanents, {Pfaffian} orientations, and even directed circuits},
     journal = {Annals of mathematics},
     publisher = {mathdoc},
     volume = {150},
     number = {3},
     year = {1999},
     url = {http://geodesic.mathdoc.fr/item/AM_1999_150_3_a4/}
}