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On the extremal number of subdivisions

Abstract:
One of the cornerstones of extremal graph theory is a result of Füredi, later reproved and given due prominence by Alon, Krivelevich, and Sudakov, saying that if H is a bipartite graph with maximum degree r on one side, then there is a constant C such that every graph with n vertices and Cn2−1/r edges contains a copy of H⁠. This result is tight up to the constant when H contains a copy of Kr,s with s sufficiently large in terms of r⁠. We conjecture that this is essentially the only situation in which Füredi’s result can be tight and prove this conjecture for r=2⁠. More precisely, we show that if H is a C4-free bipartite graph with maximum degree 2 on one side, then there are positive constants C and δ such that every graph with n vertices and Cn3/2−δ edges contains a copy of H⁠. This answers a question of Erd̋s from 1988. The proof relies on a novel variant of the dependent random choice technique which may be of independent interest.
Publication status:
Published
Peer review status:
Peer reviewed

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Publisher copy:
10.1093/imrn/rnz088

Authors


More by this author
Institution:
University of Oxford
Division:
MPLS
Department:
Mathematical Institute
Oxford college:
Wadham College
Role:
Author
ORCID:
0000-0001-5899-1829


Publisher:
Oxford University Press
Journal:
International Mathematics Research Notices More from this journal
Volume:
2021
Issue:
12
Pages:
9122–9145
Publication date:
2019-06-03
Acceptance date:
2019-04-09
DOI:
EISSN:
1687-0247
ISSN:
1687-0247


Language:
English
Keywords:
Pubs id:
pubs:987738
UUID:
uuid:e22193b2-05e5-4b70-a2b5-1717fc803203
Local pid:
pubs:987738
Source identifiers:
987738
Deposit date:
2019-04-09

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