期刊
PROCEEDINGS OF THE AMERICAN MATHEMATICAL SOCIETY
卷 145, 期 1, 页码 73-87出版社
AMER MATHEMATICAL SOC
DOI: 10.1090/proc/13221
关键词
Intersecting families; Hilton-Milner theorem; Erdos-Ko-Rado theorem
资金
- FAPESP [2014/18641-5, 2015/07869-8, 2013/03447-6, 2013/07699-0]
- CNPq [459335/2014-6, 310974/2013-5, 477203/2012-4]
- NSF [DMS 1102086]
- NUMEC/USP (MaCLinC/USP)
The celebrated Erdos-Ko-Rado theorem determines the maximum size of a k-uniform intersecting family. The Hilton-Milner theorem determines the maximum size of a k-uniform intersecting family that is not a subfamily of the so-called Erdos-Ko-Rado family. In turn, it is natural to ask what the maximum size of an intersecting k-uniform family that is neither a subfamily of the Erdos-Ko-Rado family nor of the Hilton-Milner family is. For k >= 4, this was solved (implicitly) in the same paper by Hilton-Milner in 1967. We give a different and simpler proof, based on the shifting method, which allows us to solve all cases k >= 3 and characterize all extremal families achieving the extremal value.
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