Classification of maximum hittings by large families

For integers $r$ and $n$, where $n$ is sufficiently large, and for every set $X \subseteq [n]$ we determine the maximal left-compressed intersecting families $\mathcal{A}\subseteq \binom{[n]}{r}$ which achieve maximum hitting with $X$ (i.e. have the most members which intersect $X$). This answers a question of Barber, who extended previous results by Borg to characterise those sets $X$ for which maximum hitting is achieved by the star.Comment: v2: minor corrections in response to reviewer comments. To appear in Graphs and Combinatoric

Maximum hitting for n sufficiently large

For a left-compressed intersecting family \A contained in [n]^(r) and a set X contained in [n], let \A(X) = {A in \A : A intersect X is non-empty}. Borg asked: for which X is |\A(X)| maximised by taking \A to be all r-sets containing the element 1? We determine exactly which X have this property, for n sufficiently large depending on r.Comment: Version 2 corrects the calculation of the sizes of the set families appearing in the proof of the main theorem. It also incorporates a number of other smaller corrections and improvements suggested by the anonymous referees. 7 page

Non-trivial intersecting uniform sub-families of hereditary families

For a family F of sets, let μ(F ) denote the size of a smallest set in F that is not a subset of any other set in F , and for any positive integer r, let F (r) denote the family of r-element sets in F . We say that a family A is of Hilton–Milner (HM) type if for some A ∈ A, all sets in A \ {A} have a common element x ̸∈ A and intersect A. We show that if a hereditary family H is compressed and μ(H) ≥ 2r ≥ 4, then the HM-type family {A ∈ H(r): 1 ∈ A, A∩[2,r+1] ̸= ∅}∪{[2,r+1]}is a largest non-trivial intersecting sub-family of H(r); this generalises a well-known result of Hilton and Milner. We demonstrate that for any r ≥ 3 and m ≥ 2r, there exist non-compressed hereditary families H with μ(H) = m such that no largest non-trivial intersecting sub-family of H(r) is of HM type, and we suggest two conjectures about the extremal structures for arbitrary hereditary families.peer-reviewe

A Hilton–Milner-type theorem and an intersection conjecture for signed sets

A family A of sets is said to be intersecting if any two sets in A intersect (i.e. have at least one common element). A is said to be centred if there is an element common to all the sets in A; otherwise, A is said to be non-centred. For any r ∈ [n] := {1, . . . , n} and any integer k ≥ 2, let Sn,r,k be the family {{(x1, y1), . . . , (xr, yr)}: x1, . . . , xr are distinct elements of [n], y1, . . . , yr ∈ [k]} of k-signed r-sets on [n]. Let m := max{0, 2r−n}.We establish the following Hilton–Milner-type theorems, the second of which is proved using the first: (i) If A1 and A2 are non-empty cross-intersecting (i.e. any set in A1 intersects any set in A2) sub-families of Sn,r,k, then |A1| + |A2| ≤ n R K r −r i=m r I (k − 1) I n – r r – I K r−i + 1. (ii) If A is a non-centred intersecting sub-family of Sn,r,k, 2 ≤ r ≤ n, then |A| ≤ n – 1 r – 1 K r−1 −r−1 i=m r I (k − 1) I n − 1 – r r − 1 – I K r−1−i + 1 if r < n; k r−1 − (k − 1) r−1 + k − 1 if r = n. We also determine the extremal structures. (ii) is a stability theorem that extends Erdős–Ko–Rado-type results proved by various authors. We then show that (ii) leads to further evidence for an intersection conjecture suggested by the author about general signed set systems.peer-reviewe

Compression with wildcards: All exact, or all minimal hitting sets

Our main objective is the COMPRESSED enumeration (based on wildcards) of all minimal hitting sets of general hypergraphs. To the author's best knowledge the only previous attempt towards compression, due to Toda [T], is based on BDD's and much different from our techniques. Numerical experiments show that traditional one-by-one enumeration schemes cannot compete against compressed enumeration when the degree of compression is high. Our method works particularly well in these two cases: Either compressing all exact hitting sets, or all minimum-cardinality hitting sets. It also supports parallelization and cut-off (i.e. restriction to all minimal hitting sets of cardinality at most m).Comment: 30 pages, many Table

On Complexity of 1-Center in Various Metrics

We consider the classic 1-center problem: Given a set P of n points in a metric space find the point in P that minimizes the maximum distance to the other points of P. We study the complexity of this problem in d-dimensional $\ell_p$-metrics and in edit and Ulam metrics over strings of length d. Our results for the 1-center problem may be classified based on d as follows. $\bullet$ Small d: We provide the first linear-time algorithm for 1-center problem in fixed-dimensional $\ell_1$ metrics. On the other hand, assuming the hitting set conjecture (HSC), we show that when $d=\omega(\log n)$, no subquadratic algorithm can solve 1-center problem in any of the $\ell_p$-metrics, or in edit or Ulam metrics. $\bullet$ Large d. When $d=\Omega(n)$, we extend our conditional lower bound to rule out sub quartic algorithms for 1-center problem in edit metric (assuming Quantified SETH). On the other hand, we give a $(1+\epsilon)$-approximation for 1-center in Ulam metric with running time $\tilde{O_{\epsilon}}(nd+n^2\sqrt{d})$. We also strengthen some of the above lower bounds by allowing approximations or by reducing the dimension d, but only against a weaker class of algorithms which list all requisite solutions. Moreover, we extend one of our hardness results to rule out subquartic algorithms for the well-studied 1-median problem in the edit metric, where given a set of n strings each of length n, the goal is to find a string in the set that minimizes the sum of the edit distances to the rest of the strings in the set