NP-Hardness and Fixed-Parameter Tractability of Realizing Degree Sequences with Directed Acyclic Graphs

In graph realization problems one is given a degree sequence and the task is to decide whether there is a graph whose vertex degrees match the given sequence. This realization problem is known to be polynomial-time solvable when the graph is directed or undirected. In contrast, we show NP-completeness for the problem of realizing a given sequence of pairs of nonnegative integers (representing inand outdegrees) with a directed acyclic graph (DAG), answering an open question of Berger and MüllerHannemann. Furthermore, we classify the problem as fixed-parameter tractable with respect to the parameter “maximum degree”. Investigating sparse and dense settings, we show that the problem remains NP-hard even if the realizing DAG (precisely, the underlying undirected graph) can be transformed into a clique (a tree) by adding (deleting) a constant fraction of the arcs. In contrast, if at most k arcs have to be inserted respectively removed to obtain a clique or a tree in the underlying undirected graph, then the problem becomes fixed-parameter tractable with respect to k.