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Register a new userOn generalized Turan results in height two posets
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In this paper, we show that, for all $ngeq 5$, the maximum number of $2$-chains in a butterfly-free family in the $n$-dimensional Boolean lattice is $leftlceilfrac{n}{2}rightrceilbinom{n}{lfloor n/2rfloor}$. In addition, for the height-2 poset $K_{s,t}$, we show that, for fixed $s$ and $t$, a $K_{s,t}$-free family in the $n$-dimensional Boolean lattice has $Oleft(nbinom{n}{lfloor n/2rfloor}right)$ $2$-chains.
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For a graph $H$ consisting of finitely many internally disjoint paths connecting two vertices, with possibly distinct lengths, we estimate the corresponding extremal number $text{ex}(n,H)$. When the lengths of all paths have the same parity, $text{ex}(n,H)$ is $O(n^{1+1/k^ast})$, where $2k^ast$ is the size of the smallest cycle which is included in $H$ as a subgraph. We also establish the matching lower bound in the particular case of $text{ex}(n,Theta_{3,5,5})$, where $Theta_{3,5,5}$ is the graph consisting of three disjoint paths of lengths $3,5$ and $5$ connecting two vertices.
We study Turan and Ramsey-type problems on edge-colored graphs. An edge-colored graph is called {em $varepsilon$-balanced} if each color class contains at least an $varepsilon$-proportion of its edges. Given a family $mathcal{F}$ of edge-colored graphs, the Ramsey function $R(varepsilon, mathcal{F})$ is the smallest $n$ for which any $varepsilon$-balanced $K_n$ must contain a copy of an $Finmathcal{F}$, and the Turan function $mathrm{ex}(varepsilon, n, mathcal{F})$ is the maximum number of edges in an $n$-vertex $varepsilon$-balanced graph which avoids all of $mathcal{F}$. In this paper, we consider this Turan function for several classes of edge-colored graphs, we show that the Ramsey function is linear for bounded degree graphs, and we prove a theorem that gives a relationship between the two parameters.
Motivated by generalizing Khovanovs categorification of the Jones polynomial, we study functors $F$ from thin posets $P$ to abelian categories $mathcal{A}$. Such functors $F$ produce cohomology theories $H^*(P,mathcal{A},F)$. We find that CW posets, that is, face posets of regular CW complexes, satisfy conditions making them particularly suitable for the construction of such cohomology theories. We consider a category of tuples $(P,mathcal{A},F,c)$, where $c$ is a certain ${1,-1}$-coloring of the cover relations in $P$, and show the cohomology arising from a tuple $(P,mathcal{A},F,c)$ is functorial, and independent of the coloring $c$ up to natural isomorphism. Such a construction provides a framework for the categorification of a variety of familiar topological/combinatorial invariants: anything expressible as a rank-alternating sum over a thin poset.
The extremal number $mathrm{ex}(n,F)$ of a graph $F$ is the maximum number of edges in an $n$-vertex graph not containing $F$ as a subgraph. A real number $r in [1,2]$ is realisable if there exists a graph $F$ with $mathrm{ex}(n , F) = Theta(n^r)$. Several decades ago, ErdH{o}s and Simonovits conjectured that every rational number in $[1,2]$ is realisable. Despite decades of effort, the only known realisable numbers are $0,1, frac{7}{5}, 2$, and the numbers of the form $1+frac{1}{m}$, $2-frac{1}{m}$, $2-frac{2}{m}$ for integers $m geq 1$. In particular, it is not even known whether the set of all realisable numbers contains a single limit point other than two numbers $1$ and $2$. In this paper, we make progress on the conjecture of ErdH{o}s and Simonovits. First, we show that $2 - frac{a}{b}$ is realisable for any integers $a,b geq 1$ with $b>a$ and $b equiv pm 1 ~({rm mod}:a)$. This includes all previously known ones, and gives infinitely many limit points $2-frac{1}{m}$ in the set of all realisable numbers as a consequence. Secondly, we propose a conjecture on subdivisions of bipartite graphs. Apart from being interesting on its own, we show that, somewhat surprisingly, this subdivision conjecture in fact implies that every rational number between 1 and 2 is realisable.
We discuss a possible characterization, by means of forbidden configurations, of posets which are embeddable in a product of finitely many scattered chains.
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