On the distribution of sums of residues

We generalize and solve the \roman{mod}\,q analogue of a problem of Littlewood and Offord, raised by Vaughan and Wooley, concerning the distribution of the $2^n$ sums of the form $\sum_{i=1}^n\varepsilon_ia_i$, where each $\varepsilon_i$ is $0$ or $1$. For all $q$, $n$, $k$ we determine the maximum, over all reduced residues $a_i$ and all sets $P$ consisting of $k$ arbitrary residues, of the number of these sums that belong to $P$.Comment: 5 page

Poset-free Families and Lubell-boundedness

Given a finite poset $P$, we consider the largest size \lanp of a family \F of subsets of $[n]:=\{1,...,n\}$ that contains no subposet $P$. This continues the study of the asymptotic growth of \lanp; it has been conjectured that for all $P$, \pi(P):= \lim_{n\rightarrow\infty} \lanp/\nchn exists and equals a certain integer, $e(P)$. While this is known to be true for paths, and several more general families of posets, for the simple diamond poset \D_2, the existence of $\pi$ frustratingly remains open. Here we develop theory to show that $\pi(P)$ exists and equals the conjectured value $e(P)$ for many new posets $P$. We introduce a hierarchy of properties for posets, each of which implies $\pi=e$, and some implying more precise information about \lanp. The properties relate to the Lubell function of a family \F of subsets, which is the average number of times a random full chain meets \F. We present an array of examples and constructions that possess the properties

No four subsets forming an N

AbstractWe survey results concerning the maximum size of a family F of subsets of an n-element set such that a certain configuration is avoided. When F avoids a chain of size two, this is just Sperner's theorem. Here we give bounds on how large F can be such that no four distinct sets A,B,C,D∈F satisfy A⊂B, C⊂B, C⊂D. In this case, the maximum size satisfies (n⌊n2⌋)(1+1n+Ω(1n2))⩽|F|⩽(n⌊n2⌋)(1+2n+O(1n2)), which is very similar to the best-known bounds for the more restrictive problem of F avoiding three sets B,C,D such that C⊂B, C⊂D

Diamond-free Families

Given a finite poset P, we consider the largest size La(n,P) of a family of subsets of $[n]:=\{1,...,n\}$ that contains no subposet P. This problem has been studied intensively in recent years, and it is conjectured that $\pi(P):= \lim_{n\rightarrow\infty} La(n,P)/{n choose n/2}$ exists for general posets P, and, moreover, it is an integer. For $k\ge2$ let \D_k denote the $k$-diamond poset $\{A< B_1,...,B_k < C\}$. We study the average number of times a random full chain meets a $P$-free family, called the Lubell function, and use it for P=\D_k to determine \pi(\D_k) for infinitely many values $k$. A stubborn open problem is to show that \pi(\D_2)=2; here we make progress by proving \pi(\D_2)\le 2 3/11 (if it exists).Comment: 16 page

Labeling Graphs with a Condition at Distance 2

Given a simple graph G (V, E) and a positive number d, an Ld(2, 1)-labelling of G is a function f V(G) [0, oc) such that whenever x, y E V are adjacent, If(x)- f(Y)l&gt;- 2d, and whenever the distance between x and y is two, If(x) f(Y)l&gt;- d. The Ld(2, 1)-labelling number A(G, d) is the smallest number m such that G has an Ld(2, 1)-labelling f with max{f(v) v E V} m. It is shown that to determine A(G, d), it suffices to study the case when d 1 and the labelling is nonnegative integral-valued. Let A(G) A(G, 1). The labelling numbers of special classes of graphs, e.g., A(C) 4 for any cycle C, are described. It is shown that for graphs of maximum degree A, A(G)