Complexity of Multiple-Hamiltonicity in Graphs of Bounded Degree

Brian Liu; Nathan S. Sheffield; Alek Westover

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arXiv · 2405.16270

Complexity of Multiple-Hamiltonicity in Graphs of Bounded Degree

Brian Liu·Nathan S. Sheffield·Alek Westover

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Abstract

We study the following generalization of the Hamiltonian cycle problem: Given integers $a,b$ and graph $G$, does there exist a closed walk in $G$ that visits every vertex at least $a$ times and at most $b$ times? Equivalently, does there exist a connected $[2a,2b]$ factor of $2b \cdot G$ with all degrees even? This problem is NP-hard for any constants $1 \leq a \leq b$. However, the graphs produced by known reductions have maximum degree growing linearly in $b$. The case $a = b = 1 $ -- i.e. Hamiltonicity -- remains NP-hard even in $3$-regular graphs; a natural question is whether this is true for other $a$, $b$. In this work, we study which $a, b$ permit polynomial time algorithms and which lead to NP-hardness in graphs with constrained degrees. We give tight characterizations for regular graphs and graphs of bounded max-degree, both directed and undirected.

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Brian Liu, Nathan S. Sheffield, Alek Westover. 2024-05-25. Complexity of Multiple-Hamiltonicity in Graphs of Bounded Degree. https://arxiv.org/abs/2405.16270

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