@article{10_2307_121059,
     author = {Neil Robertson and P.D. Seymour and Robin Thomas},
     title = {Permanents, {Pfaffian} orientations, and even directed circuits},
     journal = {Annals of mathematics},
     year = {1999},
     volume = {150},
     number = {3},
     doi = {10.2307/121059},
     zbl = {0947.05066},
     url = {http://geodesic.mathdoc.fr/articles/10.2307/121059/}
}