P. Frankl
2026.2.9ACTA MATHEMATICA HUNGARICA
Abstract
Let n > 2k ≥ 4 and let F ⊂ ([n] k ) be an intersecting k-graph on n vertices, that is, F ∩ F ′ ̸= ∅ for all F, F ′ ∈ F . By the Erd˝ os–Ko–Rado Theo- rem |F| ≤ (n − 1 k − 1 ) with equality holding only for the full-star, Sx, the family of all k-sets containing the vertex x. If we exclude stars, that is, subfamilies of Sx then |F| ≤ (n − 1 k − 1 ) − (n − k − 1 k − 1 ) + 1 was proved by Hilton and Milner who de- termined the families attaining equality as well. Half a cen tury later Han and Kohayakawa determined the next largest families. Then very recently Huang and Peng determined the fourth largest families. That is, th ey determined the largest families assuming that F is not a star and it is not contained neither in the Hilton–Milner families nor in the Han–Kohayakawa famil ies. In the present paper we provide a unified simple proof for these two theorems as well as solve the corresponding problem for t-intersecting families ( |F ∩ F ′| ≥ t) albeit only for n > t + max{4t(k − t + 1)2, 2(t + 1)2}.
Citation format
FRANKL, P. Concise proofs concerning the size and structure of large intersecting $$k$$-graphs. ACTA MATHEMATICA HUNGARICA, 2026.