Realization Problems on Reachability Sequences.

The classical Erdos-Gallai theorem kicked off the study of graph realizability by characterizing degree sequences. We extend this line of research by investigating realizability of directed acyclic graphs (DAGs) given both a local constraint via degree sequences and a global constraint via a sequence of reachability values (number of nodes reachable from a given node). We show that, without degree constraints, DAG reachability realization is solvable in linear time, whereas it is strongly NP-complete given upper bounds on in-degree or out-degree. After defining a suitable notion of bicriteria approximation based on consistency, we give two approximation algorithms achieving \\(O(\\log n)\\)-reachability consistency and \\(O(\\log n)\\)-degree consistency; the first, randomized, uses LP (Linear Program) rounding, while the second, deterministic, employs a k-set packing heuristic. We end with two conjectures that we hope motivate further study of realizability with reachability constraints.