@article{AM2_1980__20_182401,
     author = {L.D. Andersen and C. Thomassen},
     title = {The cover-index of infinite graphs.},
     journal = {Aequationes mathematicae},
     pages = {244--251},
     publisher = {mathdoc},
     volume = {20},
     year = {1980},
     zbl = {0429.05040},
     url = {http://geodesic.mathdoc.fr/item/AM2_1980__20_182401/}
}