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We present new quantum algorithms for Triangle Finding improving its best previously known quantum query complexities for both dense and sparse instances. For dense graphs on n vertices, we get a query complexity of O(n^(5/4)) without any of the extra logarithmic factors present in the previous algorithm of Le Gall [FOCS'14]. For sparse graphs with m >= n^(5/4) edges, we get a query complexity of O(n^(11/12) m^(1/6) sqrt(log n)), which is better than the one obtained by Le Gall and Nakajima [ISAAC'15] when m >= n^(3/2). We also obtain an algorithm with query complexity O(n^(5/6) (m log n)^(1/6) + d_2 sqrt(n)) where d_2 is the variance of the degree distribution. Our algorithms are designed and analyzed in a new model of learning graphs that we call extended learning graphs. In addition, we present a framework in order to easily combine and analyze them. As a consequence we get much simpler algorithms and analyses than previous algorithms of Le Gall based on the MNRS quantum walk framework [SICOMP'11].
@InProceedings{carette_et_al:LIPIcs.STACS.2017.20,
author = {Carette, Titouan and Lauri\`{e}re, Mathieu and Magniez, Fr\'{e}d\'{e}ric},
title = {{Extended Learning Graphs for Triangle Finding}},
booktitle = {34th Symposium on Theoretical Aspects of Computer Science (STACS 2017)},
pages = {20:1--20:14},
series = {Leibniz International Proceedings in Informatics (LIPIcs)},
ISBN = {978-3-95977-028-6},
ISSN = {1868-8969},
year = {2017},
volume = {66},
editor = {Vollmer, Heribert and Vall\'{e}e, Brigitte},
publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
address = {Dagstuhl, Germany},
URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.STACS.2017.20},
URN = {urn:nbn:de:0030-drops-70132},
doi = {10.4230/LIPIcs.STACS.2017.20},
annote = {Keywords: Quantum query complexity, learning graphs, triangle finding}
}