cross edge

cross edge:
let $\text{[math]}$ be a back edge directed from $\text{[math]}$ to $\text{[math]}$ . the edge $\text{[math]}$ crosses (called a cross edge) $\text{[math]}$ if $\text{[math]}$ is an ascendant of $\text{[math]}$ and $\text{[math]}$ is a descendant of $\text{[math]}$ (and of $\text{[math]}$ )