In hypergraphs, an edge that crosses a cut (i.e., a bipartition of nodes) can be split in several ways, depending on how many nodes are placed on each side of the cut. A cardinality-based splitting function assigns a nonnegative cost of $w_i$ for each cut hyperedge $e$ with exactly $i$ nodes on the side of the cut that contains the minority of nodes from $e$. The cardinality-based minimum $s$-$t$ cut aims to find an $s$-$t$ cut with minimum total cost. We answer a recently posed open question by proving that the problem becomes NP-hard outside the submodular region shown by~\cite{veldt2022hypergraph}. Our result also holds for $r$-uniform hypergraphs with $r \geq 4$. Specifically for $4$-uniform hypergraphs we show that the problem is NP-hard for all $w_2 > 2$, and additionally prove that the No-Even-Split problem is NP-hard. We then turn our attention to approximation strategies and approximation hardness results in the non-submodular case. We design a strategy for projecting non-submodular penalties to the submodular region, which we prove gives the optimal approximation among all such projection strategies. We also show that alternative approaches are unlikely to provide improved guarantees, by showing matching approximation hardness bounds assuming the Unique Games Conjecture and asymptotically tight approximation hardness bounds assuming $\text{P} \neq \text{NP}$.
Evaluating QAOA expectation values can be as hard as counting optimal solutions
Quantum Artificial Intelligence Laboratory (QuAIL), NASA Ames Research Center（量子人工智能实验室（QuAIL），NASA艾姆斯研究中心）；USRA Research Institute for Advanced Computer Science（美国大学空间研究协会高级计算机科学研究院）
A complexity dichotomy for Matching Cut in (bipartite) graphs of fixed diameter
在图中，匹配割是匹配的边割。匹配割是判定给定图是否有匹配割的问题，即使仅限于二分图，也是NP完全的。证明了直径为2的图的匹配割是多项式可解的。本文证明了对任意固定的整数$d\ge 3$，匹配割在直径为$d$的图类中是NP-完全的.这解决了Borowiecki和Jesse-Józefczyk在[Matching cutsets in graphs of diameter $2$，Theoretical计算机科学407（2008）574-582]中提出的公开问题。然后，我们表明，对于任何固定的整数$d\ge 4$，匹配割是NP-完全的，即使限制在类的二部图的直径$d$。作为对硬度结果的补充，我们证明了匹配割在直径不超过3的二部图类中是多项式时间可解的，并提出了一种新的求解直径为$2$的图类中匹配割的简单多项式时间算法.
