Markus Hagenbuchner Alessandro Sperduti Ah Chung Tsoi Graph self-organizing maps for cyclic and unbounded graphs. 1419-1430 2009 72 Neurocomputing 7-9 http://dx.doi.org/10.1016/j.neucom.2008.12.021 db/journals/ijon/ijon72.html#HagenbuchnerST09