On signed digraphs with all cycles negative

AbstractIn “On signed digraphs with all cycles negative”, Discrete Appl. Math. 12 (1985) 155–164, F. Harary, J.R. Lundgren and J.S. Maybee, identify certain families of such digraphs: the class of strong and upper digraphs and the class Ū. We give here a characterization of the latter class and new proofs of two results concerning these classes, by using the c-minimal strongly connected digraphs. This note answers some questions of the authors