,
Hongyang Liu
,
Yitong Yin
,
Can Zhou
Creative Commons Attribution 4.0 International license
The Coupling from the Past (CFTP) paradigm is a canonical method for perfect sampling. For uniform sampling of proper q-colorings in graphs with maximum degree Δ, the bounding chains of [Huber, STOC '98] provide a systematic framework for efficiently implementing CFTP algorithms within the classical regime q ≥ (1+o(1))Δ². This was subsequently improved to q > 3Δ by [Bhandari and Chakraborty, STOC '20] and to q ≥ (8/3 + o(1))Δ by [Jain, Sah, and Sawhney, STOC '21]. In this work, we establish the asymptotically tight threshold for bounding-chain-based CFTP algorithms for graph colorings. We prove a lower bound showing that all such algorithms satisfying the standard contraction property require q ≥ 2.5Δ, and we present an efficient CFTP algorithm that achieves this asymptotically optimal threshold q ≥ (2.5 + o(1))Δ via an optimal design of bounding chains.
@InProceedings{ding_et_al:LIPIcs.ICALP.2026.76,
author = {Ding, Tianxing and Liu, Hongyang and Yin, Yitong and Zhou, Can},
title = {{Tight Bounds for Sampling q-Colorings via Coupling from the Past}},
booktitle = {53rd International Colloquium on Automata, Languages, and Programming (ICALP 2026)},
pages = {76:1--76:24},
series = {Leibniz International Proceedings in Informatics (LIPIcs)},
ISBN = {978-3-95977-428-4},
ISSN = {1868-8969},
year = {2026},
volume = {374},
editor = {Bhattacharya, Sayan and Nanongkai, Danupon and Benedikt, Michael and Puppis, Gabriele},
publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
address = {Dagstuhl, Germany},
URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.ICALP.2026.76},
URN = {urn:nbn:de:0030-drops-264657},
doi = {10.4230/LIPIcs.ICALP.2026.76},
annote = {Keywords: perfect sampling, coupling from the past, graph coloring, bounding chains, Markov chains}
}