@article{ZNSL_2001_280_a3,
     author = {S. V. Buyalo},
     title = {Metrics of nonpositive curvature on graph-manifolds and electromagnetic fields on graphs},
     journal = {Zapiski Nauchnykh Seminarov POMI},
     pages = {28--72},
     year = {2001},
     volume = {280},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/ZNSL_2001_280_a3/}
}