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} }
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