,
Jeroen S. K. Lamme
,
Ruben F. A. Verhaegh
Creative Commons Attribution 4.0 International license
We study the parameterized complexity of a recently introduced multi-agent variant of the Kidney Exchange problem. Given a directed graph G and integers d and k, the standard problem asks whether G contains a packing of vertex-disjoint cycles, each of length ≤ d, covering at least k vertices in total. In the multi-agent setting we consider, the vertex set is partitioned over several agents who reject a cycle packing as solution if it can be modified into an alternative packing that covers more of their own vertices. A cycle packing is called rejection-proof if no agent rejects it and the problem asks whether such a packing exists that covers at least k vertices. We exploit the sunflower lemma on a set packing formulation of the problem to give a kernel for this Σ₂^P-complete problem that is polynomial in k for all constant values of d. We also provide a 2^𝒪(k log k) + n^𝒪(1) algorithm based on it and show that this FPT algorithm is asymptotically optimal under the ETH. Further, we generalize the problem by including an additional positive integer c in the input that naturally captures how much agents can modify a given cycle packing to reject it. For every constant c, the resulting problem simplifies from being Σ₂^P-complete to NP-complete. The super-exponential lower bound already holds for c = 2, though. We present an ad-hoc single-exponential algorithm for c = 1. These results reveal an interesting discrepancy between the classical and parameterized complexity of the problem and give a good view of what makes it hard.
@InProceedings{jansen_et_al:LIPIcs.IPEC.2025.9,
author = {Jansen, Bart M. P. and Lamme, Jeroen S. K. and Verhaegh, Ruben F. A.},
title = {{An ETH-Tight FPT Algorithm for Rejection-Proof Set Packing with Applications to Kidney Exchange}},
booktitle = {20th International Symposium on Parameterized and Exact Computation (IPEC 2025)},
pages = {9:1--9:15},
series = {Leibniz International Proceedings in Informatics (LIPIcs)},
ISBN = {978-3-95977-407-9},
ISSN = {1868-8969},
year = {2025},
volume = {358},
editor = {Agrawal, Akanksha and van Leeuwen, Erik Jan},
publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
address = {Dagstuhl, Germany},
URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.IPEC.2025.9},
URN = {urn:nbn:de:0030-drops-251414},
doi = {10.4230/LIPIcs.IPEC.2025.9},
annote = {Keywords: Parameterized complexity, Multi-agent kidney exchange, Kernelization, Set packing}
}