Separable convex optimization over indegree polytopes

Abstract

We study egalitarian (acyclic) orientations of undirected graphs under indegree-based objectives, such as minimizing the $φ$-sum of indegrees for a strictly convex function $φ$, decreasing minimization (dec-min), and increasing maximization (inc-max). In the non-acyclic setting of Frank and Murota (2022), a single orientation simultaneously optimizes these three objectives, however, restricting to acyclic orientations confines us to the corners of the indegree polytope, where these fairness objectives do diverge. We establish strong hardness results across a broad range of settings: minimizing the $φ$-sum of indegrees is NP-hard for every discrete strictly convex function $φ$; dec-min and inc-max are NP-hard for every indegree bound $k \geq 2$, as well as without a bound; and the complementary inc-min and dec-max problems are NP-hard even on $3$-regular graphs. On the algorithmic side, we give a polynomial-time algorithm for minimizing the maximum weighted indegree via a weighted smallest-last ordering. We also provide an exact exponential-time algorithm for minimizing general separable discrete convex objectives over indegrees, and a polynomial-time algorithm for the non-acyclic case. Finally, for maximizing the sum of the products of indegrees and outdegrees, we prove NP-hardness on graphs of maximum degree $4$, give an algorithm for maximum degree $3$, and provide a $3$-approximation algorithm. Our results delineate the algorithmic frontier of convex integral optimization over indegree (base-)polytopes, and highlight both theoretical consequences and practical implications, notably for scheduling and deadlock-free routing.