Gabriel Nivasch’s “multi-stack” algorithm identifies these structures. An unsolved problem that asks whether these structures produce a double cover reduces to asking about “snarks.” These structures cause Kahn’s algorithm to output an empty list, since they prevent topological sortings. Pollard’s (10[1])algorithm for integer factorization is named for the eventual resemblance to one of these structures over time. Gosper’s algorithm for detecting these structures improves upon another algorithm that creates a leading and trailing pointer called a “hare” and “tortoise” and is named for Floyd. Detecting (10[1])these structures by checking already-visited nodes ensures that DFS (10[1])halts. The [emphasize] absence of these structures defines the “A” in a DAG. For 10 points, name these paths in a graph that (10[1])eventually lead back to their start. ■END■