HartsfieldR89'

@article{DBLP:journals/jct/HartsfieldR89,
  author    = {Nora Hartsfield and
               Gerhard Ringel},
  title     = {Minimal quadrangulations of nonorientable surfaces},
  journal   = {J. Comb. Theory, Ser. A},
  volume    = {50},
  number    = {2},
  year      = {1989},
  pages     = {186-195},
  ee        = {http://dx.doi.org/10.1016/0097-3165(89)90014-9},
  bibsource = {DBLP, http://dblp.uni-trier.de}
}