In one of my algorithm courses, there is this:
A subset $S$ of vertices in a directed graph $G$ is strongly connected if for each pair of distinct vertices ($v_i$, $v_j$) in $S$, $v_i$ is connected to $v_j$ and $v_j$ is connected to $v_i$.
And then the following example graph is given for this proposition:
Maybe i do not understand what that phrase means. What i think it means is this: A node, say $E$ can be a member of an ordered pair $(v_i, v_j)$ only once, ie. if $(E, A)$, then $\lnot (E, [someOtherNode]) $. But we clearly see here, that we have $(E,A)$ and $(E,D)$.
How should i correctly interpret this phrase. What does it mean exactly? Thanks.