,
Aleksey Lopez
Creative Commons Attribution 4.0 International license
We study multiplicative graph spanners in the f-degree fault tolerant (f-DFT) model, in which the spanner must approximately preserve distances even after any subset of edges of maximum degree f temporarily "fails" and is removed from the graph. We prove that there are n-node lower bound graphs for which any f-DFT (2k-1)-stretch spanner H must have size |E(H)| ≥ Ω(f^{1-1/k} n^{1+1/k}) . This matches a lower bound that was previously only known to hold conditionally, under the 1963 girth conjecture of Erdős. It also matches the current upper bounds, up to a factor of exp(k). Our proof is an analysis of the so-called Wenger graphs (J. Comb. Theory 1991), via their recent reinterpretation by Szabó and by Conlon (Am. Math. Monthly 2021).
@InProceedings{bodwin_et_al:LIPIcs.ESA.2026.31,
author = {Bodwin, Greg and Lopez, Aleksey},
title = {{Unconditional Lower Bounds for Degree Fault Tolerant Spanners}},
booktitle = {34th Annual European Symposium on Algorithms (ESA 2026)},
pages = {31:1--31:13},
series = {Leibniz International Proceedings in Informatics (LIPIcs)},
ISBN = {978-3-95977-445-1},
ISSN = {1868-8969},
year = {2026},
volume = {388},
editor = {Bille, Philip and Pettie, Seth and Storandt, Sabine},
publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
address = {Dagstuhl, Germany},
URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.ESA.2026.31},
URN = {urn:nbn:de:0030-drops-271673},
doi = {10.4230/LIPIcs.ESA.2026.31},
annote = {Keywords: Spanners, Fault Tolerance, Girth Conjecture, Wenger Graphs}
}