Making an oriented graph acyclic using inversions of bounded or prescribed size

TL;DR

This paper studies the problem of making an oriented graph acyclic through bounded size vertex inversions, providing polynomial algorithms for most cases, bounds on inversion numbers, and complexity results including NP-hardness and kernelization.

Contribution

It introduces a polynomial-time decision algorithm for $(=p)$-invertibility when $p eq n-1$, establishes bounds on inversion numbers, and analyzes the complexity of related decision problems.

Findings

Deciding $(=n-1)$-invertibility is NP-complete.

Polynomial algorithms exist for $(=p)$-invertibility when $p eq n-1$.

NP-hardness and $W[1]$-hardness results are established for related problems.

Abstract

Given an oriented graph $D$, the inversion of a subset $X$ of vertices consists in reversing the orientation of all arcs with both endpoints in $X$. When the subset $X$ is of size $p$ (resp. at most $p$), this operation is called an $(=p)$-inversion (resp. $(\leq p)$-inversion). Then, an oriented graph is $(=p)$-invertible if it can be made acyclic by a sequence of $p$-inversions. We observe that, for $n=|V(D)|$, deciding whether $D$ is $(=n-1)$-invertible is equivalent to deciding whether $D$ is acyclically pushable, and thus NP-complete. In all other cases, when $p \neq n-1$, we construct a polynomial-time algorithm to decide $(=p)$-invertibility. We then consider the $(= p)$-inversion number, $\text{inv}^{= p}(D)$ (resp. $(\leq p)$-inversion number, $\text{inv}^{\leq p}(D)$), defined as the minimum number of $(=p)$-inversions (resp. $(\leq p)$-inversions) rendering $D$ acyclic. We show that every $(=p)$-invertible digraph $D$ satisfies $\text{inv}^{= p}(D) \leq |A(D)|$ for every integer $p\geq 2$. When $p$ is even, we bound $\text{inv}^{= p}$ by a (linear) function of the feedback arc set number, and rule out the existence of any bounding function for odd $p$. Finally, we study the complexity of deciding whether the $(= p)$-inversion number, or the $(\leq p)$-inversion number, of a given oriented graph is at most a given integer $k$. For any fixed positive integer $p \geq 2$, when $k$ is part of the input, we show that both problems are NP-hard even in tournaments. In general oriented graphs, we prove $W[1]$-hardness for both problems when parameterized by $p$, even for $k=1$. In contrast, we exhibit polynomial kernels in $p + k$ for both problems in tournaments.