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Register a new userStability for Intersecting Families of Perfect Matchings
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Nathan Lindzey
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A family of perfect matchings of $K_{2n}$ is $intersecting$ if any two of its members have an edge in common. It is known that if $mathcal{F}$ is family of intersecting perfect matchings of $K_{2n}$, then $|mathcal{F}| leq (2n-3)!!$ and if equality holds, then $mathcal{F} = mathcal{F}_{ij}$ where $ mathcal{F}_{ij}$ is the family of all perfect matchings of $K_{2n}$ that contain some fixed edge $ij$. In this note, we show that the extremal families are stable, namely, that for any $epsilon in (0,1/sqrt{e})$ and $n > n(epsilon)$, any intersecting family of perfect matchings of size greater than $(1 - 1/sqrt{e} + epsilon)(2n-3)!!$ is contained in $mathcal{F}_{ij}$ for some edge $ij$. The proof uses the Gelfand pair $(S_{2n},S_2 wr S_n)$ along with an isoperimetric method of Ellis.
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A family of perfect matchings of $K_{2n}$ is $t$-$intersecting$ if any two members share $t$ or more edges. We prove for any $t in mathbb{N}$ that every $t$-intersecting family of perfect matchings has size no greater than $(2(n-t) - 1)!!$ for sufficiently large $n$, and that equality holds if and only if the family is composed of all perfect matchings that contain a fixed set of $t$ disjoint edges. This is an asymptotic version of a conjecture of Godsil and Meagher that can be seen as the non-bipartite analogue of the Deza-Frankl conjecture proven by Ellis, Friedgut, and Pilpel.
The extremal problems regarding the maximum possible size of intersecting families of various combinatorial objects have been extensively studied. In this paper, we investigate supersaturation extensions, which in this context ask for the minimum number of disjoint pairs that must appear in families larger than the extremal threshold. We study the minimum number of disjoint pairs in families of permutations and in $k$-uniform set families, and determine the structure of the optimal families. Our main tool is a removal lemma for disjoint pairs. We also determine the typical structure of $k$-uniform set families without matchings of size $s$ when $n ge 2sk + 38s^4$, and show that almost all $k$-uniform intersecting families on vertex set $[n]$ are trivial when $nge (2+o(1))k$.
A family of sets is said to be emph{symmetric} if its automorphism group is transitive, and emph{intersecting} if any two sets in the family have nonempty intersection. Our purpose here is to study the following question: for $n, kin mathbb{N}$ with $k le n/2$, how large can a symmetric intersecting family of $k$-element subsets of ${1,2,ldots,n}$ be? As a first step towards a complete answer, we prove that such a family has size at most [expleft(-frac{c(n-2k)log n}{k( log n - log k)} right) binom{n}{k},] where $c > 0$ is a universal constant. We also describe various combinatorial and algebraic approaches to constructing such families.
We shall be interested in the following Erdos-Ko-Rado-type question. Fix some subset B of [n]. How large a family A of subsets of [n] can we find such that the intersection of any two sets in A contains a cyclic translate (modulo n) of B? Chung, Graham, Frankl and Shearer have proved that, in the case where B is a block of length t, we can do no better than to take A to consist of all supersets of B. We give an alternative proof of this result, which is in a certain sense more direct.
Let $mathcal{F}$ and $mathcal{G}$ be two $t$-uniform families of subsets over $[k] = {1,2,...,k}$, where $|mathcal{F}| = |mathcal{G}|$, and let $C$ be the adjacency matrix of the bipartite graph whose vertices are the subsets in $mathcal{F}$ and $mathcal{G}$, and there is an edge between $Ain mathcal{F}$ and $B in mathcal{G}$ if and only if $A cap B eq emptyset$. The pair $(mathcal{F},mathcal{G})$ is $q$-almost cross intersecting if every row and column of $C$ has exactly $q$ zeros. We consider $q$-almost cross intersecting pairs that have a circulant intersection matrix $C_{p,q}$, determined by a column vector with $p > 0$ ones followed by $q > 0$ zeros. This family of matrices includes the identity matrix in one extreme, and the adjacency matrix of the bipartite crown graph in the other extreme. We give constructions of pairs $(mathcal{F},mathcal{G})$ whose intersection matrix is $C_{p,q}$, for a wide range of values of the parameters $p$ and $q$, and in some cases also prove matching upper bounds. Specifically, we prove results for the following values of the parameters: (1) $1 leq p leq 2t-1$ and $1 leq q leq k-2t+1$. (2) $2t leq p leq t^2$ and any $q> 0$, where $k geq p+q$. (3) $p$ that is exponential in $t$, for large enough $k$. Using the first result we show that if $k geq 4t-3$ then $C_{2t-1,k-2t+1}$ is a maximal isolation submatrix of size $ktimes k$ in the $0,1$-matrix $A_{k,t}$, whose rows and columns are labeled by all subsets of size $t$ of $[k]$, and there is a one in the entry on row $x$ and column $y$ if and only if subsets $x,y$ intersect.
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