Abstract
Let n be a positive integer, q a power of a prime, and \(\mathcal {L}_{n}({q})\) the poset of subspaces of an n-dimensional vector space over a field with q elements. This poset is a normalized matching poset and the set of subspaces of dimension ⌊n/2⌋ or those of dimension ⌈n/2⌉ are the only maximum-sized antichains in this poset. Strengthening this well-known and celebrated result, we show that, except in the case of \(\mathcal {L}_{3}({2})\), these same collections of subspaces are the only maximum-sized families in \(\mathcal {L}_{n}({q})\) that avoid both a ∧ and a ∨ as a subposet. We generalize some of the results to brooms and forks, and we also show that the union of the set of subspaces of dimension k and k + 1, for k = ⌊n/2⌋ or k = ⌈n/2⌉ − 1, are the only maximum-sized families in \(\mathcal {L}_{n}({q})\) that avoid a butterfly (definitions below).
Similar content being viewed by others
References
Anderson, I.: Combinatorics of Finite Sets. Dover Publications, Mineola (2002). Corrected reprint of the 1989 edition published by Oxford University Press, Oxford
Kirby, A., Baker, A: generalization of Sperner’s lemma. J. Combinatorial Theory Ser. A 6(2), 224–225 (1969)
De Bonis, A, Katona, G.O.H., Swanepoel, K.J.: Largest family without A∪B ⊆C∩D. J. Combin. Theory Ser. A 111(2), 331–336 (2005)
Engel, K.: Sperner Theory, Encyclopedia of Mathematics. Cambridge University Press, Cambridge (1997)
Gerbner, D.: The covering lemma and q-analogues of extremal set theory problems, arXiv:1905.06994v2
Gerbner, D, Methuku, A, Nagy, D.T., Patkós, B., Vizer, M: Forbidding rank-preserving copies of a poset. Order. https://doi.org/10.1007/s11083-019-09484-5(2019)
Graham, R.L., Harper, L.H.: Some results on matching in bipartite graphs. SIAM J. Appl. Math. 17, 1017–1022 (1969)
Griggs, J.R., Li, W.-T., Lu, L.: Diamond-free families. J. Combin. Theory Ser. A 119(2), 310–322 (2012)
Grósz, D., Methuku, A., Tompkins, C.: An upper bound on the size of diamond-free families of sets. J. Combin. Theory Ser. A 156, 164–194 (2018)
Katona, G.O.H.: Forbidden Intersection Patterns in the Families of Subsets (Introducing a Method), Horizons of Combinatorics, Bolyai Soc. Math. Stud., vol. 17, pp 119–140. Springer, Berlin (2008)
Katona, G.O.H., Tarján, T.G.: Extremal Problems with Excluded Subgraphs in the n-Cube, Graph Theory (Łagów, 1981), Lecture Notes in Math., vol. 1018, pp 84–93. Springer, Berlin (1983)
Kleitman, D.J.: On an extremal property of antichains in partial orders. The LYM property and some of its implications and applications. Combinatorics (Proc. NATO Advanced Study Inst., Breukelen Part 2: Graph theory; foundations, partitions and combinatorial geometry. Math. Centrum, Amsterdam, pp. 77–90. Math. Centre Tracts, No. 56 (1974)
Methuku, A., Tompkins, C.: Exact forbidden subposet results using chain decompositions of the cycle. Electron. J. Combin. 22(4), 14 (2015). Paper 4.29
Nagy, D.T.: Forbidden subposet problems with size restrictions. J. Combin. Theory Ser. A 155, 42–66 (2018)
Salerno, P.: Forbidden Posets, Undergraduate Thesis. Pomona College, Claremont (2009)
Sarkis, G, Shahriari, S, PCURC: Diamond-free subsets in the linear lattices, vol. 31. PCURC stands for the Pomona College Undergraduate Research Circle whose members in Spring of 2011 were Zachary Barnett, David Breese, Benjamin Fish, William Frick, Andrew Khatutsky, Daniel McGuinness, Dustin Rodrigues, and Claire Ruberman (English)
Sperner, E: Ein satz über Untermengen einer endlichen Mengen. Math. Z. 27, 544–8 (1928)
van Lint, J.H., Wilson, R.M.: A Course in Combinatorics, 2nd edn. Cambridge University Press, Cambridge (2001)
Xiao, J, Tompkins, C.: On forbidden poset problems in the linear lattice, arXiv:1905.09246v1
Acknowledgments
The authors thank Pomona College’s Summer Undergraduate Research Program for supporting the second author.
Author information
Authors and Affiliations
Corresponding author
Additional information
Publisher’s Note
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Rights and permissions
About this article
Cite this article
Shahriari, S., Yu, S. Avoiding Brooms, Forks, and Butterflies in the Linear Lattices. Order 37, 223–242 (2020). https://doi.org/10.1007/s11083-019-09501-7
Received:
Accepted:
Published:
Issue Date:
DOI: https://doi.org/10.1007/s11083-019-09501-7