OPODIS 2022 · 25th International Conference on Principles of Distributed Systems, Brussels, Belgium, December 2022 · doi:10.4230/LIPIcs.OPODIS.2022.21

In this paper, we study the notion of mending, i.e. given a partial solution to a graph problem, we investigate how much effort is needed to turn it into a proper solution. For example, if we have a partial coloring of a graph, how hard is it to turn it into a proper coloring?

In prior work (SIROCCO 2022), this question was formalized and studied from the perspective of *mending radius*: if there is a hole that we need to patch, how *far* do we need to modify the solution? In this work, we investigate a complementary notion of *mending volume*: how *many* nodes need to be modified to patch a hole?

We focus on the case of locally checkable labeling problems (LCLs) in trees, and show that already in this setting there are two infinite hierarchies of problems: for infinitely many values $0 < \alpha \le 1$, there is an LCL problem with mending volume $\Theta(n^\alpha)$, and for infinitely many values $k \ge 1$, there is an LCL problem with mending volume $\Theta(\log^k n)$. Hence the mendability of LCL problems on trees is a much more fine-grained question than what one would expect based on the mending radius alone.

We define three variants of the theme: (1) *existential* mending volume, i.e., how many nodes need to be modified, (2) *expected* mending volume, i.e., how many nodes we need to explore to find a patch if we use randomness, and (3) *deterministic* mending volume, i.e., how many nodes we need to explore if we use a deterministic algorithm. We show that all three notions are distinct from each other, and we analyze the landscape of the complexities of LCL problems for the respective models.

Eshcar Hillel, Roberto Palmieri, and Etienne Rivière (Eds.): *26th International Conference on Principles of Distributed Systems (OPODIS 2022)*, volume 253 of *Leibniz International Proceedings in Informatics (LIPIcs)*, pages 21:1–21:17, Schloss Dagstuhl–Leibniz-Zentrum für Informatik, 2023

ISBN 978-3-95977-265-5