Directionally 2-signed and bidirected graphs. (English) Zbl 1301.05161
Summary: An edge \(uv\) in a graph \(\Gamma\) is directionally 2-signed (or, \((2,d)\)-signed) by an ordered pair \((a,b)\), \(a, b \in \{+,-\}\), if the label \(l(uv) = (a,b)\) from \(u\) to \(v\), and \(l(vu) = (b,a)\) from \(v\) to \(u\). Directionally 2-signed graphs are equivalent to bidirected graphs, where each end of an edge has a sign. A bidirected graph implies a signed graph, where each edge has a sign. We extend a theorem of Sriraj and Sampathkumar by proving that the signed graph is antibalanced (all even cycles and only even cycles have positive edge sign product) if, and only if, in the bidirected graph, after suitable reorientation of edges every vertex is a source or a sink.