Aiping Deng Iwao Sato Yaokun Wu Characteristic polynomials of ramified uniform covering digraphs. 1099-1114 2007 28 Eur. J. Comb. 4 http://dx.doi.org/10.1016/j.ejc.2006.04.001 db/journals/ejc/ejc28.html#DengSW07