With \(A=\{1,3,5\}\) and \(B=\{2,4,6\}\), the edges \(\{1,4\}\), \(\{2,5\}\) and \(\{3,6\}\) join different parts. But \(\{1,3\}\) closes the triangle \(1,2,3\).
A bipartite graph is a graph \(G=(V,E)\) whose vertices split into two disjoint sets \(A\) and \(B\) so that every edge \(e\in E\) joins a vertex of \(A\) to one of \(B\).
\[V=A\cup B,\quad A\cap B=\varnothing,\quad |e\cap A|=1\ \text{ for every } e\in E\]