Abstract
The fastest known classical algorithm deciding the k-colorability of n-vertex graph requires running time \(\Omega (2^n)\) for \(k\ge 5\). In this work, we present an exponential-space quantum algorithm computing the chromatic number with running time \(O(1.9140^n)\) using quantum random access memory (QRAM). Our approach is based on Ambainis et al.’s quantum dynamic programming with applications of Grover’s search to branching algorithms. We also present a polynomial-space quantum algorithm not using QRAM for the graph 20-coloring problem with running time \(O(1.9575^n)\). For the polynomial-space quantum algorithm, we essentially show \((4-\epsilon )^n\)-time classical algorithms that can be improved quadratically by Grover’s search.
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Notes
- 1.
In this paper, \(O^*(f(n))\) means \(O(\mathrm {poly}(n)f(n))\).
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Acknowledgment
This work was supported by JST PRESTO Grant Number JPMJPR1867 and JSPS KAKENHI Grant Numbers JP17K17711 and JP18H04090. The authors thank François Le Gall for the insightful comments.
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Shimizu, K., Mori, R. (2020). Exponential-Time Quantum Algorithms for Graph Coloring Problems. In: Kohayakawa, Y., Miyazawa, F.K. (eds) LATIN 2020: Theoretical Informatics. LATIN 2021. Lecture Notes in Computer Science(), vol 12118. Springer, Cham. https://doi.org/10.1007/978-3-030-61792-9_31
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