,
François Dross
,
Cyril Gavoille
Creative Commons Attribution 4.0 International license
We consider the problem of finding the smallest graph that contains two input trees each with at most n vertices preserving their distances. In other words, we look for an isometric-universal graph with the minimum number of vertices for two given trees. We prove that this problem can be solved in time O(n^{5/2}log{n}). We extend this result to forests instead of trees, and propose an algorithm with running time O(n^{7/2}log{n}). As a key ingredient, we show that a smallest isometric-universal graph of two trees essentially is a tree. Furthermore, we prove that these results cannot be extended. Firstly, we show that deciding whether there exists an isometric-universal graph with t vertices for three forests is NP-complete. Secondly, we show that any smallest isometric-universal graph cannot be a tree for some families of three trees. This latter result has implications for greedy strategies solving the smallest isometric-universal graph problem.
@InProceedings{baucher_et_al:LIPIcs.MFCS.2025.16,
author = {Baucher, Edgar and Dross, Fran\c{c}ois and Gavoille, Cyril},
title = {{Isometric-Universal Graphs for Trees}},
booktitle = {50th International Symposium on Mathematical Foundations of Computer Science (MFCS 2025)},
pages = {16:1--16:16},
series = {Leibniz International Proceedings in Informatics (LIPIcs)},
ISBN = {978-3-95977-388-1},
ISSN = {1868-8969},
year = {2025},
volume = {345},
editor = {Gawrychowski, Pawe{\l} and Mazowiecki, Filip and Skrzypczak, Micha{\l}},
publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
address = {Dagstuhl, Germany},
URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.MFCS.2025.16},
URN = {urn:nbn:de:0030-drops-241237},
doi = {10.4230/LIPIcs.MFCS.2025.16},
annote = {Keywords: tree, forest, isometric subgraph, universal graph, distance-preserving}
}